Recent progress in cadmium fluorescent and colorimetric probes

Abstract

Cadmium is a heavy metal which exists widely in industrial and agricultural production and can induce a variety of diseases in organisms. Therefore, its detection is of great significance in the fields of biology, environment and medicine. Fluorescent probe has been a powerful tool for cadmium detection because of its convenience, sensitivity, and bioimaging capability. In this paper, we reviewed 98 literatures on cadmium fluorescent sensors reported from 2017 to 2021, classified them according to different fluorophores, elaborated the probe design, application characteristics and recognition mode, summarized and prospected the development of cadmium fluorescent and colorimetric probes. We hope to provide some help for researchers to design cadmium fluorescent probes with higher selectivity, sensitivity and practicability.

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Chun-tian Shi

Chun-Tian Shi was born in Yunnan, P. R. China in 1996. She received her B. S. degree from Yunnan Agricultural University in 2019. Then, she entered Yangtze University for her M. A. degree and joined Ai-Bin Wu's research group. She was awarded the excellent graduate student titled by the Graduate School of Yangtze University in 2020. Her research interests focuses on the design, synthesis and application of metal ion-targeted fluorescent probes.

Ai-bin Wu

Ai-Bin Wu was born in Hubei, P. R. China in 1973. He received his B. S. degree from Central China Normal University in 1995 and obtained his PhD degree in Applied Chemistry with Dr Xu-Hong Qian from East China University of Science and Technology in 2010. He joined Dr Chang-Guo Zhan's group as a Visiting Scientist at College of Pharmacy, the University of Kentucky. In 2015, he began his independent career as associate professor at School of Chemistry and Environmental Engineering, Yangtze University. His research interests are highly interdisciplinary and include functional dyes, synthesis and application of fine chemicals, medicinal chemistry.

Wei-chu Yu

Wei-Chu Yu was selected as the National “Hundred, Thousand, Ten Thousand” talent project candidate in 2014 and the distinguished professor at Yangtze University. He attended China University of Petroleum (Beijing) to receive his PhD degree in 2006. After that he performed postdoctoral research at China University of Petroleum (Beijing) under the direction of Dr Chang-Ming Su, he joined Dr Yong-Chun Tang's and Ce Liu's group as senior Visiting Scientist at California Energy Research Institute and University of Houston, respectively. Prof. Yu's research involves the synthesis and application of fine chemicals, especially in applied chemistry of oil and gas fields.

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1. Introduction

Cadmium, as an essential resource in the earth, is widely used in chemical industry, electronics industry, nuclear industry, semiconducting, quantum dots, phosphate fertilizers, rechargeable batteries, pigments, electroplating, stabilizer, metallurgy, ceramic enamels.^1–6 Consequently, there is a widespread Cd²⁺ contamination in air, water, and soil.⁷ Cadmium is a non-essential substance for the organism, and is even classified as a human carcinogen.^8–10 Chronic exposure to Cd²⁺ may cause renal dysfunction, cardiovascular diseases, lung disease, calcium metabolism disorders, eosinophilia, neurodegenerative diseases.^3,11–14 According to World Health Organization (WHO), the permissible concentration of Cd²⁺ in drinking water is 3 μg L⁻¹.^15,16 Therefore, the detection of Cd²⁺ content in organisms, food, and the environment is very important and urgent for life and health.^17,18

Up to now, there are some traditional detection methods of Cd²⁺ include atomic absorption spectroscopy, ultraviolet-visible spectroscopy, ion selective electrode, inductively coupled plasma mass spectrometry, stripping voltammetry.^19–23 Nevertheless, most of these detection methods require expensive equipment, tedious procedures and skilled operators, and have the characteristics of low detection sensitivity, high economic costs and time-consuming.^18,23–25 By contrast, fluorescent probe detection has the characteristics of strong specificity, high sensitivity, low detection limit, high accuracy, low cost, and visualization, which can replace traditional analysis methods to detect Cd²⁺.^{15,16,26–28} Therefore, the research and application of Cd²⁺ fluorescent sensors is of great interest to many scientific fields, ranging from supramolecular chemistry to life sciences.

Fluorescent sensors consist of fluorophore (signalling) and receptor (guest binding) moieties, either separated by a spacer or integrated into one unit.^29,30 Common fluorophores are rhodamine, naphthalimide, coumarin, quinolone, BODIPY, benzothiazole, fluorescein, diarylethylene.^{4,16,17,19,21,22,31–34} Common receptors are pyridine and its analogues, and other structures containing O, N, S and other heteroatoms.⁹ According to the generation of fluorescence signals, there are mainly the following mechanisms: photoinduced electron transfer (PET), intramolecular charge transfer (ICT), fluorescence resonance energy transfer (FRET), aggregation-induced emission (AIE), excimer/exciplex formation/extinction, C=N isomerization, excited-state intramolecular proton transfer (ESIPT). In addition, Cd²⁺ fluorescent and colorimetric nanosensors, including metal organic framework (MOF), quantum dots (QDs), nanoclusters (NCs) and nanoparticles (NPs), have also achieved good development in recent years, which makes it more possibilities for the source of raw materials, structural design, recognition methods and application fields of Cd²⁺ probes.

Trace Cd²⁺ is harmful to environment and organism, and the detection of Cd²⁺ ions by fluorescent probes is easily interfered by other transition metals, especially Zn²⁺ ions in the same group.^3,21 Therefore, the development of Cd²⁺ fluorescent probes has great challenges and significance for the detection of Cd²⁺ in environment and living organisms. At present, a plethora of fluorescent probes for Cd²⁺ have been reported and this research field has become very active. A closer scrutiny of the review literatures on the research progress of Cd²⁺ fluorescent probes in the web of science core collection shows that J. Jia et al. reviewed Zn²⁺/Cd²⁺ fluorescent probes based on small organic molecules in 2012, and S. Y. Chen et al. reviewed Pb²⁺/Cd²⁺/Hg²⁺ fluorescent probes based on small organic molecules in 2021.^35,36 Apart from these, there is no relevant or targeted review article that has covered research advances of Cd²⁺ fluorescent probes. Therefore, as a review article dedicated to the research progress of Cd²⁺ fluorescent probes, this article adopts a classification method different from the above reviews. Since the mechanism of some fluorescence response is inferred by the authors and has not been demonstrated absolutely, and there may be two or more possible mechanisms might simultaneously exist in the sensing process, we described the probes based on the category of fluorophore species. It should be noted that this article elaborated and summarized the Cd²⁺ ion fluorescent probes reported in recent years in terms of the probe design ideas, application characteristics, and binding methods, and discussed the reality and future challenges of Cd²⁺ ion fluorescent probes used in environmental monitoring and biological imaging.

2. Quinolone-based Cd²⁺ fluorescent sensor

Quinoline is a kind of fluorescent chromophore with coordination function, recognition function, strong photostability and metal ion chelating ability.⁹ The conjugated system of quinoline is large and prone to π–π* electronic transition. Its own N atom easily forms hydrogen bonds in polar solvents and exhibits weak fluorescence. After complexing with metal ions, the fluorescence recovers.

2.1 Quinolone as a binding site for Cd²⁺

Y. Mikata's team reported on a series of fluorescent probes (Fig. 1) for detecting Cd²⁺, and quinoline chromophores in these probes can also be used as binding sites.^1,37,38 In 2017, they synthesized probe 1a and its thia (1b) and aza (1c) derivatives.¹ In DMF–H₂O (1 : 1) solution and an excitation at 317 nm, the probes 1a, 1b and 1c exhibit weak fluorescence. The binding of the probes to Cd²⁺ inhibits their PET process and make the adjacent quinoline rings in the probes formed an intramolecular excimer, so these probes exhibit enhanced fluorescence emission. The detection limit of probe 1a for Cd²⁺ is 22 nM. The dissociation constant of the complexes of probe 1a, 1b, 1c and Cd²⁺ were (4.2 ± 0.7) × 10⁻⁷ M, (6.5 ± 0.2) × 10⁻⁶ M and <2 × 10⁻⁷ M, respectively. The O in probe 1a is replaced by S and N can increase the selectivity of Cd²⁺. For the detection of Cd²⁺ by these probes, although the fluorescence response interference caused by Zn²⁺ can be neglected, the presence of Cu²⁺, Ag⁺, Hg²⁺, Cr³⁺ and Fe³⁺ in the detection system can interfere with the fluorescence signal obviously.

Fig. 1 The probes structure reported by Y. Mikata's team and proposed binding mode with Cd²⁺.

In 2019, they designed methoxy-substituted tetrakisquinoline EGTA and BAPTA analogs probes that can recognize Cd²⁺ based on PET mechanism.³⁷ All EGTQ derivatives bind to Zn²⁺, Cd²⁺, Fe³⁺, Co²⁺, Hg²⁺ and Ag⁺ as indicated by the UV-vis spectral changes. However, only Zn²⁺ and Cd²⁺ can illuminate the ligand. Although they selectivity is poor, methoxy substitution can enhance the selectivity to Cd²⁺ to some extent. TriMeOBAPTQ probe was synthesized by introducing three methoxy groups at positions 5, 6, and 7 of each quinoline moiety in BAPTQ. In methanol–HEPES buffer (9 : 1, 50 mM HEPES, 0.1 M KCl, pH = 7.5), upon excitation at 347 nm, TriMeOBAPTQ can combine with Cd²⁺ in a stoichiometric ratio of 1 : 1 or 1 : 2 to increase the fluorescence of the system, and detect Cd²⁺ as low as 9.9 nM. TriMeOBAPTQ also has affinity with Cu²⁺, Ag⁺, Hg²⁺, and Fe³⁺, even the presence of a large number of these metal ions except Cu²⁺ does not affect the detection of Cd²⁺ by TriMeOBAPTQ probe.

To improve the metal ion specificity and sensitivity, they changed core structure from BAPTA to Ca²⁺-specific probe (quin2(8-amino-2-((2-amino-5-methylphenoxy)methyl)-6-methoxyquinoline-N,N,N′,N′-tetraacetic acid)) to synthesize probe 2, and reported it in 2020.³⁸ Probe 2 has two chromophores, methoxy-substituted and unsubstituted quinolines. Because the aqueous solvent prevents the probe 2 from coordinating with metal ions, the test was carried out in methanol solution. The combination of probe 2 and Cd²⁺ can inhibit the PET process and produce CHEF (chelation-enhanced fluorescence) and excimer effects when excited by 317 nm light, which increased the fluorescence intensity of the probe 2 by 170 times at 408 nm. The detection limit of probe 2 for Cd²⁺ is 182 nM. The equimolar amounts of Co²⁺ and Ag⁺ shut down the Cd²⁺-induced fluorescence, and the excessive Zn²⁺ and Ni²⁺ can replace Cd²⁺ to bind to probe 2. Although the probe has enhanced fluorescence and specific selection for Cd²⁺ by introducing 6-methoxy-8-aminoquioline moiety, the sensor still has weak water solubility, low selectivity, and short excitation and emission wavelengths. Moreover, the detection of low concentrations of Cd²⁺ should prevent weak fluorescent ML₂ is generated. Y. Mikata et al. proposed to introduce a suitable electron-donating substituents on the side-arm quinolones, which act as a metal binding site and chromophore to improve sensor performance.³⁸

In 2017, the sulfonamidoquinoline-based derivatives sensor 3 (Fig. 2) was reported by Y. Zhang et al.³⁹ In DMSO–water (3 : 2 (v/v), 0.01 M Tris–HCl, pH = 7.24) solution, sensor 3 and Zn²⁺/Cd²⁺/Hg²⁺ are combined in a mole ratio of 2 : 1 to red-shift the absorption maximum from 323 nm to 425/422/424 nm. When excited at 422 nm, the detection system including sensor 3 and Zn²⁺/Cd²⁺/Hg²⁺ emits red fluorescence at 634/618/630 nm, with an intensity increase of 38/96/21 times. In addition, the fluorescence intensity is linearly related to the concentration of Zn²⁺/Cd²⁺/Hg²⁺. The detection limit of sensor 3 to Zn²⁺ is 3.61 nM, to Cd²⁺ is 1.55 nM, and to Hg²⁺ is 7.02 nM, which is lower than the permissible concentrations of Cd²⁺ (43.8 nM) and Hg²⁺ (9.9 nM) in drinking water regulated by Environmental Protection Agency (EPA). The binding constants (log K_s) decreased with the increase of ion radius in group IIB, and the binding constants of Zn²⁺, Cd²⁺ and Hg²⁺ and sensor 3 were 9.45, 9.36 and 8.96, respectively. The sensor 3 can detect Zn²⁺, Cd²⁺ and Hg²⁺ in the aqueous solution under the excitation of visible light. In addition, the sensor 3 was used for Cd²⁺ competition experiments and fluorescence imaging in yeast cells, indicating that the sensor 3 is biocompatible and not affected by metal ions except Zn²⁺ and Cu²⁺ in the fluorescence detection of Cd²⁺.

Fig. 2 Proposed binding mode of probes 3 to 6 with Cd²⁺.

Y. P. Dai and coworkers described probe 4 (Fig. 2).⁹ In Tris–HCl buffer (10 mM, pH = 7.4), the detection limit to Cd²⁺ is 1.18 × 10⁻⁶ M and the association constants is 9.00 × 10⁴ M⁻¹. Upon excitation at 312 nm, probe 4 has weak fluorescence due to the double PET process from the oxygen atom of hydroxy to hydroxyquinoline group and the hydroxyquinoline to carbonyl unit, respectively. After the chelation of probe 4 with Cd²⁺, the PET process was inhibited, causing the fluorescence of probe 4 emission peak to redshift from 400 nm to 410 nm and the fluorescence to increase by 2.5 times. Although probe 4 is almost 100% soluble in water, reversibility, non-toxic and cell-membrane-permeable, the presence of Fe³⁺, Fe²⁺, Cu²⁺,Cr³⁺, Al³⁺ or Ag⁺ ions can quench part of the fluorescence enhancement caused by Cd²⁺ to some extent. At 410 nm, the fluorescence intensity of the Cd²⁺ complex probe 4 is very little affected by other metal ions, but Fe³⁺ can partially quench the fluorescence.

X. H. Ding's research group reported sensor 5 (Fig. 2).⁴⁰ In CH₃CN/H₂O (1 : 1) systems, sensor 5 combined with Cd²⁺ made the solution turn dark yellow immediately, and turn blue fluorescence under 365 nm UV light. The association constant of sensor 5 and Cd²⁺ is estimated to be 8.48 × 10⁴ M⁻¹. In the pH range of 6–10, sensor 5 has high selectivity, anti-interference ability and bioimaging ability. It can detect Cd²⁺ as low as 4 × 10⁻⁶ mol L⁻¹ and not be interfered by other cations. In addition, the complex of sensor 5 and Cd²⁺ can be used as a highly selective and sensitive probe for PO₄³⁻ without interference from anions.

In 2017, X. Y. Liu's team reported two turn-on fluorescent sensors 6a and 6b (Fig. 2) for Zn²⁺/Cd²⁺, and their complexes can act as highly selective sensors to detect phosphate anion through turn-off the fluorescence.¹¹ In CH₃OH/H₂O (1 : 1, v/v, Tris 10 mol L⁻¹, pH = 7.4) solution, upon the excitation at 310 nm, sensor 6a/6b has weak fluorescence. Adding Zn²⁺/Cd²⁺ into the sensor solution, an obvious fluorescence turn-on response at 430 nm, and a working curve can be established between the fluorescence intensity and the concentration. The binding constants of sensors 6a and 6b to Zn²⁺ or Cd²⁺ are (1.517 ± 0.31) × 10⁷, (6.40 ± 0.15) × 10⁶, (9.16 ± 0.42) × 10⁵, and (1.03 ± 0.19) × 10⁶ L mol⁻¹, respectively. Adding anion into the complexes of sensor 6a/6b and Zn²⁺/Cd²⁺, and it is found that H₂PO₄⁻ and HPO₄²⁻ can almost quench all fluorescence. However, the article does not specifically describe the distinction between Zn²⁺ and Cd²⁺, H₂PO₄⁻ and HPO₄²⁻.

In 2019, H. H. Song's research group synthesized probe 7 (Fig. 3) by inserting an amide group into the 8-aminoquinoline fluorophore and a propargylamine chelating site.⁴¹ Probe 7 displayed selective and distinct ratiometric fluorescence response to Zn²⁺ and Cd²⁺, Zn²⁺ was bound as an amide tautomer in almost totally water solution, and Cd²⁺ was bound as an imidic acid tautomer in CH₃CN aqueous medium, respectively. Under 365 nm light, in the high/low acetonitrile content aqueous solution, probe 7 and Cd²⁺/Zn²⁺ combined 1 : 1, making probe 7 emitted bright-green/green fluorescence and the emission peak is red-shifted 95/82 nm. The fluorescence intensity ratios of I_{500 nm}/I_{405 nm} and I_{498 nm}/I_{416 nm} were linearly proportional to the concentration of Cd²⁺ and Zn²⁺, respectively. The association constants of probe 7 with Cd²⁺ and Zn²⁺ are is 3.7 × 10⁴ M⁻¹ and 1.4 × 10⁴ M⁻¹, respectively. The detection limits for Cd²⁺ is 0.055 μM and for Zn²⁺ is 0.063 μM. The response of probe 7 to Zn²⁺ and Cd²⁺ is selective, reversible, and rapid. The coexistence Cu²⁺, Ni²⁺, Co²⁺ with Cd²⁺ quenched the Cd²⁺-induced fluorescence, while the coexistence Cr³⁺ with Zn²⁺ slightly quenched the Zn²⁺-induced fluorescence. Probe 7 was applied for detecting analysis of all the target ions in the tap water sample and on test paper strips, and bioimaging of Zn²⁺ in mung bean sprouts as well. In addition, the complex of probe 7 and Zn²⁺ can be used as a secondary sensor for PPi and ATP.

Fig. 3 Proposed binding mode of probes 7 to 9 with Cd²⁺.

The same year, Z. N. Lu et al. synthesized 8-hydroxyquinoline-benzothiazole conjugated probe 8 (Fig. 3) in two steps.⁴² This probe can greatly enhance the fluorescence by coordinating with various metal ions such as Al³⁺, Cd²⁺, Zn²⁺, Mg²⁺ in methanol containing 1% water. However, the selectivity to Cd²⁺ can be achieved by increasing the water content to 30% aqueous methanol solution. The aqueous methanol solution (pH = 7.4, 30% Tris–HCl buffer) and the excitation at 313 nm are selected to carry out. Probe 8 and Cd²⁺ combined at a 2 : 1 stoichiometry between probe 8 and Cd²⁺ to emit green fluorescence, and the fluorescence intensity at 525 nm had a good linear relationship with the concentration of Cd²⁺ over a range of 0–5 μM. The detection limit for Cd²⁺ is 0.1 μM. Adding EDTA as a competing chelator can make the binding of probe 8 and Cd²⁺ reversible. Probe 8 can detect Cd²⁺ in an aqueous solution with a pH of 4–12, and be used for fluorescence imaging of Cd²⁺ in living cells.

The group headed by X. J. Wan described a quinoline Schiff base-containing sensor 9 (Fig. 3).³³ After optimizing the detection environment, the researchers found that the better selectivity and sensitivity of the probe to Cd²⁺ under the conditions of pH = 4 methanol solution (10%) and excitation wavelength of 246 ± 2 nm. The combination of non-fluorescent sensor 9 and Cd²⁺ caused sensor 9 to turn on the fluorescence response at 425 nm. At this time, sensor 9 displayed excellent selectivity, sensitivity, and reversibility for detecting Cd²⁺ in an acidic environment, and it can detect Cd²⁺ as low as 2.4 nM.

In 2019, K. Aich and colleagues reported on quinoline–benzothiazole-based fluorescent turn-on probes 10 and 11 (Fig. 4).⁴³ In MeOH/H₂O (3/7, v/v, 10 mM HEPES buffer, pH = 7.2) solution, upon the excitation at 360 nm, the fluorescence emission intensity of probes 10 and 11 are linearly related to the concentration of Cd²⁺. Their detection limits for Cd²⁺ are 3.52 × 10⁻¹⁰ M and 4.83 × 10⁻⁹ M, respectively, and the association constants are 3.17 × 10⁴ M⁻¹ and 1.60 × 10⁴ M⁻¹, respectively. Because the nitrogen atoms of quinoline and pyridine are strongly coordinated with Cd²⁺, the ICT effect of the probes is enhanced. The fluorescence emission of probe 10 is red-shifted from 488 nm to 507 nm and the fluorescence is changed from weak cyan to green. The fluorescence emission of probe 11 is increased by 20 times, the fluorescence emission is red-shifted from 490 nm to 510 nm and the fluorescence is changed from cyan to bright green. Probe 10 and probe 11 have high selectivity and reversibility (Na₂EDTA) for Cd²⁺ detection, and are not interfered by other metal ions, and can act as a potential portable kit for detection of Cd²⁺ in solid state as well as in solution.

Fig. 4 The structure of probes 10 to 12.

The next year, S. Nazerdeylami et al. described probe 12 (Fig. 4), which is effective for detecting Cd²⁺ and tetracycline.⁴⁴ The 8-hydroxyquinoline dehydrogenation is connected to β-cyclodextrin, which increases the rigidity of the probe 12. In an aqueous medium, upon excitation at 350 nm, probe 12 has a strong fluorescence emission intensity at 525 nm. The N in the pyridine ring and the O in the chelator interacted with Cd²⁺, causing the fluorescence of the probe 12 to be quenched. The fluorescence intensity is linearly related to the Cd²⁺ content of 0.1–1.5 nM. At pH < 4, the solution hydrates H⁺ ions due to the protonation of nitrogen and oxygen, causing H⁺ and other metal ions to compete chelating sites. At pH > 4, cadmium hydroxide precipitates is formed. Therefore, the probe 12 is suitable for detecting Cd²⁺ in an aqueous medium with pH = 4, and the detection has selectivity, sensitivity, and anti-interference. The detection limit of probe 12 for Cd²⁺ is 0.05 nM. Further, the complex of probe 12 and Cd²⁺ can be used as a fluorescence-on probe of tetracycline, the fluorescence intensity is linearly related to the content of tetracycline, and can detect tetracycline as low as 0.9 μM.

S. L. Li et al. Reported the probe 13 (Fig. 5) responding to Ag⁺ (PET) and Cd²⁺ (ICT) based on different mechanisms.⁴⁵ In the CH₃OH/HEPES (9 : 1, v/v, pH = 7.30) buffer system and the excitation wavelength of 344 nm, the fluorescence of the probe was quenched by 88% with the addition of Ag⁺ and showed an “on–off” behavior. With the addition of Cd²⁺, the maximum fluorescence emission of the probe was red shifted from 465 nm to 490 nm, the fluorescence intensity was quenched by 33%, and the fluorescence changed from blue to green. The recognition of Cd²⁺ is not affected by metal ions including Ag⁺, while the recognition of Ag⁺ ions is interfered by other metal ions. The selectivity and sensitivity of the probe for Cd²⁺ ion are stronger than that for Ag⁺. The complexation constant of the probe for Cd²⁺ is 2.23 × 10⁴ M⁻¹, and the detection limit for Cd²⁺ is 0.26 mM. The complex of probe 13 and Cd²⁺ could be used for sequential recognition of S²⁻. When Cd²⁺ and S²⁻ were added alternately for 3 times, the probe still had a high level of recognition ability. Under strong acidic or alkaline conditions, probe 13 may change its own structure or lose the ability of ion coordination and cannot recognize ions.

Fig. 5 Proposed binding mode of probe 13 with Cd²⁺.

2.2 Quinolone does not as a binding site for Cd²⁺

In 2017, the first metal ion sensor 14 (Fig. 6) of 4,5-quinolinimide derivatives was synthesized by Y. Zhang et al.⁷ To increase the selectivity of 2,2-dipicolylamine (DPA) for Cd²⁺ over the same group element Zn²⁺, an amide group was introduced into the DPA unit. In a methanol–water (1 : 1) solution with a pH of 7.20, upon excitation at 430 nm, adding Cd²⁺ increased the fluorescence of sensor 14, and the fluorescence intensity was linearly related to the concentration of Cd²⁺. The complex's association constant is 9.06 × 10⁷ M⁻¹. The detection limit of sensor 14 for Cd²⁺ is 11 nM, which is lower than the permissible concentration of Cd²⁺ in drinking water regulated by the World Health Organization as 26 nM. The fluorescence emission of sensor 14 binding to Zn²⁺ is weaker and the fluorescent lifetime is shorter, which makes it possible to distinguish between Zn²⁺ and Cd²⁺. Under physiological conditions, the fluorescence of sensor 14 binding to Cd²⁺ can be significantly quenched by Cu²⁺. In addition, sensor 14 has low cytotoxicity and emits bright yellow-green fluorescence when used to fluorescence imaging of Cd²⁺ in yeast cells.

Fig. 6 Proposed binding mode of probes 14 to 17 with Cd²⁺.

In 2019, P. G. Mahajan's team reported the Schiff base probe 15 (Fig. 6).⁶ The probe can quickly detect Cd²⁺, Co²⁺, Ni²⁺, Cu²⁺ in nano-molar level through absorption or fluorescence spectroscopy. Adding Cd²⁺, Ni²⁺, Cu²⁺, Co²⁺ to the systems of methanol : water (7 : 3, v/v) and probe 15 for 3–5 minutes causes the probe solution to change from colorless to pale yellow, yellow, yellow, and dark yellow, and the absorption peaks red-shifts from 338 nm to 399 nm, 422 nm, 424 nm, 396 nm, respectively. The detection limits of probe 15 for Cd²⁺ is 1.043 nM, for Ni²⁺ is 0.656 nM, for Cu²⁺ is 0.224 nM, for Co²⁺ is 1.047 nM. Upon excitation at 340 nm, the complex of probe 15 with Cd²⁺, Ni²⁺, Co²⁺, Cu²⁺ absorb the energy from light followed by immediately return to ground state without photon emission, resulting in fluorescence quenching by 8.1, 9.2, 8.6, 10.5 times, respectively. The detection limits are 1.070 nM for Cd²⁺, 0.637 nM for Ni²⁺, 1.053 nM for Co²⁺, and 0.184 nM for Cu²⁺. In the aqueous solution with a pH of 6–8, and the affinity of probe 15 for metal ions is Cd²⁺ < Co²⁺ < Ni²⁺ < Cu²⁺, and the metal ions with strong affinity can replace metal ions with weak affinity to bind the probe 15.

Y. Xiao et al. designed the probe 16 (Fig. 6) by connecting quinoline fluorophore with tripyridine recognition group.⁴⁶ Due to the strong absorption at 470 nm and the elimination of the interference of the original fluorescence in DMF–H₂O (FW 40% v/v) solution, they were selected as the conditions for the study of the probe recognition performance. At 520 nm, there was a good linear relationship between the fluorescence emission intensity and the concentration of Cd²⁺ (0–5 μmol L⁻¹) and Zn²⁺ (6–10 μmol L⁻¹). The LOD of this fluorescent probe 16 for Cd²⁺ is 3.5 × 10⁻⁸ mol L⁻¹. However, the presence of Zn²⁺ and Cu²⁺ affects the recognition of Cd²⁺ by the probe. Because the fluorescence of this probe induced by Zn²⁺ is stronger than that induced by Cd²⁺, the binding of Cu²⁺ to the probe is more stable and can quench the fluorescence induced by Cd²⁺.

A. Garau et al. synthesized probe 17 (Fig. 6) by inserting amide group spacer between quinoline fluorophore and 1-aza-4,7,10-trithiacyclododecane ([12]aneNS₃) receptor unit.⁴⁷ In MeCN/H₂O (1 : 4, v/v) solution, the probe showed fluorescence enhancement in response to Cd²⁺ and Zn²⁺ due to CHEF effect, and Cd²⁺ induced fluorescence enhancement was stronger than that caused by Zn²⁺.

2.3 Binding mode remains unknown

In 2018, an “on–off–on” sensor 18 (Fig. 7) for sequential recognition of Cu²⁺ and Cd²⁺ reported by J. Han et al.⁴⁸ In EtOH/H₂O (v/v = 1 : 9) solution, sensor 18 emits strong blue fluorescence at 471 nm (the excitation at 340 nm), adding Cd²⁺ decreased the fluorescence intensity and red-shifted 40 nm, and the fluorescence changed from blue to green. Cu²⁺ binds to sensor 18 at a ratio of 1 : 1, quenching the blue fluorescence of sensor 18. The detection limit for Cu²⁺ is 2.7 × 10⁻⁸ M, and the association constant is 5.643 × 10⁴ M⁻¹. The complex of sensor 18 and Cu²⁺ can be replaced by Cd²⁺ and emerge a strong emission peak at 510 nm to become a more sensitive and accurate fluorescent probe of Cd²⁺, which significantly increases the fluorescence intensity of the system. The complex of sensor 18-Cu can detect Cd²⁺ as low as 1.7 × 10⁻⁸ M, and the association constant is 1.374 × 10⁴ M⁻¹. Sensor 18 is stable in the pH range from 4 to 10, and can successively quantitatively detect Cu²⁺ and Cd²⁺ in real water samples and deproteinized milk. Test strips containing sensor 18 also has a good qualitative recognition performance for target ions.

Fig. 7 The structure of probe 18.

Among the Cd²⁺ fluorescent probes reported in recent five years, the research quinolone-based sensors is the most, and the spectroscopic and analytical parameters of these probes are shown in Table 1. Quinoline is widely used in the design of Cd²⁺ fluorescent probes because it can not only be used as fluorescent chromophore, but also as Cd²⁺ binding site. However, most of these sensors have some problems, such as poor water solubility, poor selectivity, poor anti-interference, short excitation and emission wavelength, which greatly limits their application in the fields of environment and biology. Compared with other sensors, sensors 9 and 18-Cu have higher selectivity, sensitivity and anti-interference ability. However, the optimum test condition of sensor 9 is pH = 4, which is not suitable for the application in real environment. The sensor 18-Cu responds to Cd²⁺ by replacing Cu²⁺ with Cd²⁺, which makes the detection of Cd²⁺ more accurate. It is not be limited to the small molecule probe directly used for Cd²⁺ detection, therefore, it can also be used for Cd²⁺ detection after the probe is complexed with other analytes in the future.

Table 1 Spectroscopic and analytical parameters of the quinolone-based Cd²⁺ fluorescent probes

Probe	Solvent	Excitation wavelength (nm)	Emission wavelength (nm) λem0–λem	Detection limit (nM)	Association constant (M^−1)	Interfering ion(s)	Ref.
1a	DMF–H2O (1 : 1)	317	428	22	Kd =(4.2 ± 0.7) × 10^−7 M	Cu^2+, Ag^+, Hg^2+, Cr^3+	1
TriMeO-BAPTQ	CH3OH–H2O (9 : 1)	347	460	9.9	—	Cu^2+	37
2	CH3OH	317	408	182	—	Co^2+, Ag^+, Zn^2+, Ni^2+	38
3	DMSO–H2O (3 : 2)	422	618	1.55	log Ks = 9.36	Zn^2+, Cu^2+	39
4	H2O	312	410	1180	9.00 × 10^4	Fe^3+	9
5	CH3CN–H2O (1 : 1)	375	500	4000	8.48 × 10^4	PO4^3−	40
6a/6b	CH3OH–H2O (1 : 1)	310	430	—	(1.517 ± 0.31) × 10^7/(1.03 ± 0.19) × 10^6	Zn^2+, H2PO4^−, HPO4^2−	11
7	CH3CN–H2O	321	405–500	55	3.7 × 10^4	Cu^2+, Ni^2+, Co^2+	41
8	CH3OH–H2O (7 : 3)	313	525	100	—	—	42
9	CH3OH–H2O (1 : 9)	246	425	2.4	—	—	33
10	CH3OH–H2O (3 : 7)	360	488–507	0.352	3.17 × 10^4	—	43
11	CH3OH–H2O (3 : 7)	360	490–510	4.83	1.60 × 10^4	—	43
12	H2O	350	525	0.05	—	Tetracycline	44
13	CH3OH–H2O (9 : 1)	344	465–490	260 000	2.23 × 10^4	—	45
14	CH3OH–H2O (1 : 1)	430	543	11	9.06 × 10^7	Cu^2+, Zn^2+	7
15	CH3OH–H2O (7 : 3)	340	About 420	1.070	6.73 × 10^5	Co^2+, Ni^2+, Cu^2+	6
16	DMF-H2O (6 : 4)	470	520	35	—	Zn^2+ and Cu^2+	46
17	MeCN–H2O (1 : 4)	330	505	—	—	Zn^2+	12
18-Cu	EtOH/H2O (1 : 9)	340	510	17	1.374 × 10^4	—	48

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3. Coumarin-based Cd²⁺ fluorescent sensor

Coumarins are characterized by high molar extinction coefficient, high fluorescence quantum yield, large stokes shift, low toxicity, good water solubility and many reaction sites. In addition, coumarin is easy to connect to different groups, and can not only act as a fluorescent chromophore, but also as recognition sites of analytes, so as to achieve fluorescence response to different analytes.^34,49

3.1 Coumarin as a binding site for Cd²⁺

In 2017, C. Kumari's research team synthesized probe 19 (Fig. 8), which recognizes Cd²⁺ and turns on fluorescence.² There is a spectral overlap between the energy donor coumarin part and the energy acceptor rhodamine part, and Cd²⁺ is recognized based on the FRET mechanism. In methanol/water (2/1, v/v, 1 mM HEPES buffer, pH = 7.2), the free probe has no characteristic absorption peak in the visible light region, but the absorption peak intensity at 561 nm is increased after gradually adding Cd²⁺. Upon the addition of Cd²⁺, the probe 19 and Cd²⁺ are combined in a ratio of 1 : 1 to open the spirolactam ring. The solution immediately changed from colorless to pink, turned non-fluorescent to bright red fluorescent under long wavelength UV light, and shifted the fluorescence emission peak from 430 nm to 589 nm (the excitation wavelength at 340 nm). The binding constant value of the probe to Cd²⁺ was 7.8 × 10⁵ M⁻³ and the detection level was 10.1 nM. Probe 19 can detect Cd²⁺ in a semi-aqueous environment with a pH of 1.0–11.0, with excellent selectivity and sensitivity, and is not interfered by other metal ions. Further, bio-imaging study and cytotoxicity test confirm that it can be used for detecting Cd²⁺ in living HeLa S3 cells.

Fig. 8 Proposed binding mode of probes 19 to 23 with Cd²⁺.

In 2017, Shaily et al. synthesized the coumarin-chalcone conjugated probe 20 (Fig. 8).⁵⁰ In HEPES-buffered solution (20 mM, CH₃CN : H₂O, 3 : 7, v/v, pH = 7.0), probe 20 and Cd²⁺ follows a 1 : 1 binding ratio, making the solution immediately change from yellow to colorless and the absorption maximum blue-shift from 387 nm to 370 nm, and the absorbance has a good linear relationship with concentration of Cd²⁺. When excited at 387 ± 3 nm, Cd²⁺ made the almost non-fluorescent probe 20 emit light blue fluorescence at 495 nm, and the fluorescence lifetime increase from 0.217 ns to 1.97 ns. According to Benesi–Hildebrand plot and nonlinear least squar fitting, the association constant for Cd²⁺ towards probe 20 were 9.56 × 10⁵ M⁻¹ and (1.34 ± 0.87) × 10⁶ M⁻¹, respectively. Probe 20 does not respond to metal ions except Cd²⁺, it can be used as a Cd²⁺ colorimetric and turn-on fluorescence probe. In mixed aqueous-organic media with the pH range of 7.0–9.0, it can detect Cd²⁺ as low as 5.84 × 10⁻⁸ M. Moreover, the test strip containing probe 20 can sense Cd²⁺.

In 2018, K. Krishnaveni et al. reported a ratiometric probe 21 (Fig. 8) that based on ICT mechanism to recognize Cd²⁺/F⁻.³ In CH₃CN/H₂O (1 : 9, v/v) buffered with HEPES (pH = 7.52), the probe 21 is combined with Cd²⁺/F⁻ at a ratio of 1 : 1. Upon the excitation wavelength at 340 ± 5 nm, the fluorescence of probe 21 was changed by Cd²⁺ from pale yellow to dark yellow, and the fluorescence intensity ratio of I_{418 nm}/I_{530 nm} was linearly related to the concentration of Cd²⁺. The fluorescence of probe 21 can also be changed by F⁻, and the fluorescence intensity ratio I_{415 nm}/I_{530 nm} can quantitatively detect F⁻. The binding constants of probe 21 for Cd²⁺ and F⁻ ions were 3.51 × 10⁻³ M and 5.34 × 10⁻³ M, respectively. Probe 21 can be used as a high-selectivity and high-sensitivity proportional fluorescence sensor for Cd²⁺ and F⁻ with detection limits of 1.5 × 10⁻¹⁰ mol L⁻¹ and 1.2 × 10⁻¹⁰ mol L⁻¹, respectively. In addition, probe 21 has reversibility (EDTA) and low cytotoxicity, and can be used for fluorescence imaging of Cd²⁺ in H9c2 cancer cells.

S. Zehra's team described coumarin derived turn-on probe 22 (Fig. 8) in 2019.⁴⁹ The spectral studies were carried out in a THF/H₂O (1 : 1) solution and an excitation at 300 ± 5 nm. Probe 22 chelating Cd²⁺ inhibited C=N isomerization of probe, so the fluorescence of probe 22 increased and the fluorescence emission intensity is linearly related to the Cd²⁺ of 1–15 μM. The probe 22 was combined with Cd²⁺ at 1 : 1 stoichiometry, the K_a was 3.3 × 10⁵ M⁻¹, and the detection limit for Cd²⁺ was 0.114 μM. The response of probe 22 to Cd²⁺ is rapid and reversible (EDTA), without obvious interference from other metal ions, and can quantitatively detect the micro molar concentration of Cd²⁺ in the environmental and biological samples as well.

In 2019, Y. F. Tang's study group reported probe 23 (Fig. 8), in which coumarin is a fluorophore and tri-(2-aminoethyl)-amine is a selective recognition unit for Cd²⁺.³⁴ In CH₃CN–HEPES (90 : 10, v/v, pH = 7.4) solution, the probe 23 binding to Cd²⁺ inhibited its C=N isomerization and PET process. When excited by 361 nm, probe 23 showed that the blue fluorescence was turned on, the fluorescence quantum yield increased, and the fluorescence emission peak shifted from 505 nm to 457 nm. Further, the fluorescence intensity was linearly related to the amount of Cd²⁺. The probe 23 bound to Cd²⁺ in a 1 : 2 ratio, the association constants and detection limits were 1.37 × 10¹¹ M⁻² and 1.16 × 10⁻⁷ M, respectively. The probe 23 recognizes Cd²⁺ well and is only slightly interfered by Zn²⁺. However, since EDTA makes probe 23 reversibly bind to Zn²⁺ and irreversibly bind to Cd²⁺, the effect of Zn²⁺ can be eliminated. Therefore, it can be used as a high selectivity and sensitivity probe to detect Cd²⁺. And it also can be as an imaging agent for fluorescence imaging of Cd²⁺ in HepG-2 cells.

The spectroscopic and analytical parameters of the coumarin-based Cd²⁺ fluorescent probes are compiled in Table 2. These probes recognizes Cd²⁺ according to different response mechanisms, and its value in practical application has been proved by test paper test, reversibility, pH application range and biological imaging. However, optimizing its water solubility, sensitivity, selectivity and anti-interference is still a problem to be solved in future research.

Table 2 Spectroscopic and analytical parameters of the coumarin-based Cd²⁺ fluorescent probes

Probe	Solvent	Excitation wavelength (nm)	Emission wavelength (nm) λem0–λem	Detection limit (nM)	Association constant	Interfering ion(s)	Ref.
19	CH3OH–H2O (2 : 1)	340	430–589	10.1	7.8 × 10^5 M^−3	—	2
20	CH3CN–H2O (3 : 7)	387	495	58.4	(1.34 ± 0.87) × 10^6 M^−1	—	50
21	CH3CN–H2O (1 : 9)	340	418	0.15	3.51 × 10^−3 M	F^−	3
22	THF–H2O (1 : 1)	300	About 360	114	3.3 × 10^5 M^−1	—	49
23	CH3CN–H2O (9 : 1)	361	505–457	116	1.37 × 10^11 M^−2	Zn^2+	34

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4. Benzothiazole based Cd²⁺ fluorescent sensor

Benzothiazole is an important type of heterocyclic ring containing N and S, which can be used as a fluorophore and is widely used in the design and construction of ion recognition probes.^21,24 In some cases, the N and S contained in it can also be used as a coordination site for ions.

In 2018, J. Li et al. described sensor 24 (Fig. 9).⁴ The DMF/H₂O (9 : 1, v/v) solution and the excitation at 411 nm were selected as the exploration conditions. Due to the combined action of the ESIPT effect from the 2-(2-hydroxyphenyl)-benzothiazole moiety and AIE effect, sensor 24 exhibited weak orange fluorescence at 573 nm. The combination of sensor 24 with Zn²⁺ or Cd²⁺ inhibits the ESIPT effect and the PET process, thereby increasing the fluorescence emission, changing color from orange to yellow, and blue-shifting the fluorescence emission peak from 573 nm to 520 nm or 540 nm, respectively. The fluorescence intensity of the sensor 24 combined with Zn²⁺ or Cd²⁺ has a linear relationship with the concentration of Zn²⁺ or Cd²⁺. The detection limits of sensor 24 for Zn²⁺ and Cd²⁺ are 0.036 μM, 1.16 μM, respectively. The binding constants are 3.27 × 10⁴ M⁻¹ and 2.93 × 10³ M⁻¹, respectively. The addition of cysteine makes the complex of sensor 24 and Cd²⁺ restore the fluorescence signal of sensor 24, while the complex of sensor 24 and Zn²⁺ only reduces the fluorescence intensity. Thus, cysteine can be used as an auxiliary agent in the probe detection of Zn²⁺ and Cd²⁺ to distinguish them. Moreover, sensor 24 can be used for optical cell imaging of Zn²⁺ or Cd²⁺ and on-site analysis of test paper.

Fig. 9 Proposed binding mode of probes 24 to 26 with Cd²⁺.

The same year, R. Diana's research group designed probe 25 (Fig. 9) based on the ICT mechanism to respond to Cd²⁺.²⁴ In ethanol–water solution, the probe 25 can quantitatively detect Fe³⁺ and Fe²⁺ through naked eye. Adding Zn²⁺/Cd²⁺, the fluorescence intensity of probe 25 increased by 3.25/4.35 times and the emission peak red-shifted from 348 nm to 370/375 nm. The detection limits for Zn²⁺ and Cd²⁺ were 205 nM and 642 nM, respectively. The binding constant for Zn²⁺ and Cd²⁺ were log K_a = 4.10 and 4.25, respectively. To understand the mode of binding to the sensor, the complex of Zn²⁺ and the sensor is studied by X-ray crystallography, and the combination mode is shown in Fig. 10. However, due to the low selectivity, sensitivity, and anti-interference (such as: Fe³⁺, Fe²⁺, Co²⁺), the probe 25 is difficult to meet the detection requirements.

Fig. 10 The structure of probes 27 to 30.

In 2021, S. Paul et al. reported a multifunctional naphthalene-benzothiazole based sensor 26 (Fig. 9).⁵¹ The donor part such as –C=N enhances the reliability of the sensor, and the active functional group such as –OH improves the color detection efficiency of the sensor for the target analyte. Sensor 26 (1 × 10⁻⁵ M, CH₃CN) can recognize Cd²⁺ in pure water by turning on fluorescence. When excited at 390 nm, due to the –C=N isomerization, PET, and ESIPT mechanism, the sensor exhibits weak green fluorescence emission at 505 nm. The combination of Cd²⁺ with sensor 26 increases the molecular rigidity, inhibits the effects of PET and ESIPT, and promotes the CHEF process, which leads to a significant increase in the fluorescence intensity of this sensor. The association constant (K_a) of the sensor with Cd²⁺ is 0.91 × 10⁶ M^−1/2, and the detection limit for Cd²⁺ is 16.7 nM. This sensor can also detect Zn²⁺ or CN⁻ in pure water medium. In addition, sensor 26 has cell membrane permeability and biocompatibility, and can detect the content of Cd²⁺ and CN⁻ in bitter almonds and yeast cells.

Like quinoline and coumarin, benzothiazole can be used not only as fluorophore, but also as Cd²⁺ chelating site. We provide the spectroscopic and analytical parameters of these probes benzothiazole-based fluorophore in Table 3. These probes are responsive to multiple analytes and are also disturbed by a variety of ions. Among them, the probe 24 can recognize Zn²⁺ and Cd²⁺ at the same time. The research team uses cysteine as an auxiliary agent to distinguish Zn²⁺ and Cd²⁺, which is also a way to eliminate interference and roughly identify Cd²⁺ in the environment. However, the quantitative detection of Cd²⁺ still needs a probe with high selectivity and anti-interference.

Table 3 Spectroscopic and analytical parameters of the benzothiazole-based Cd²⁺ fluorescent probes

Probe	Solvent	Excitation wavelength (nm)	Emission wavelength (nm) λem0–λem	Detection limit (nM)	Association constant (Ka)	Interfering ion(s)	Ref.
24	DMF–H2O (9 : 1)	411	573–540	1160	2.93 × 10^3 M^−1	Cysteine, Zn^2+	4
25	EtOH–H2O	309	348–375	642	log Ka = 4.25	Fe^3+, Fe^2+, Co^2+, Zn^2+	24
26	CH3CN	390	505	16.7	0.91 × 10^6 M^−1/2	Zn^2+, CN^−	51

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5. Rhodamine-based Cd²⁺ fluorescent sensor

Rhodamine derivatives possess high fluorescence quantum yield, photostability, large molar extinction coefficient, bioavailability, and its excitation wavelengths and emission wavelengths in the visible light region.^23,31 Furthermore, it exist an equilibrium between the nonfluorescent “spirolactam ring” and fluorescent “ring-open” forms to allow analyte sensing through “off–on” switching.^23,31,52 Therefore, it is widely used in the design of fluorescent sensors.

In 2017, P. Sakthivel's research group synthesized a rhodamine based senor 27 (Fig. 10) by linking to diaminomaleonitrile moiety.⁵² This sensor can not only identify Cd²⁺ by naked eyes, but also can detect Cd²⁺ quantitatively by UV-vis absorption and fluorescence spectra. Upon addition of Cd²⁺, an absorption peak emerged at 530 nm and the intensity had a linear relationship with Cd²⁺ concentration. Upon complexation, colourless spirolactam form is converted into colored ring opened amide form, which displays a noticeable naked-eye detection of the magenta. In HEPES buffer (acetonitrile–water = 7 : 3, 10 μM, pH = 7.54), sensor 27 was combined with Cd²⁺ in a ratio of 1 : 1, which increased its weak fluorescence emission at 553 nm by 200 times (excitation at 530 nm). When the Cd²⁺ concentration was 1.0 × 10⁻⁷ to 1.0 × 10⁻⁵ mol L⁻¹, the fluorescence intensity [1/(F − F₀)] at 553 nm had a linear relationship with 1/[Cd²⁺]. The association constant between sensor 27 and Cd²⁺ was 2.33 × 10⁵ M⁻¹, and the detection limit of sensor 27 for Cd²⁺ was 18.5 nM. It can be used for the determination of Cd²⁺ in river and tap water samples and as an imaging agent for Cd²⁺ in living cells.

M. Maniyazagan et al. designed a rhodamine pyridine conjugated probe 28 (Fig. 10) based on FRET mechanism, which can recognize Cd²⁺ by colorimetry and fluorescence.³² In ACN/HEPES buffer (2 : 8, V/V, pH = 7.2, 10 mM), the probe was excited at 308 nm. With the addition of Cd²⁺, probe 28 showed orange yellow fluorescence, the maximum emission wavelength red-shifted from 480 nm to 590 nm, and the solution changed from colorless to magenta. The recognition of Cd²⁺ by this probe is reversible (S²⁻) and not interfered by other metal ions. In addition, probe 28 can also be used for imaging in HeLa cells at physiological pH. The binding constant of probe 28 to Cd²⁺ is 4.2524 × 10⁴ M⁻¹, and the detection limit is 1.025 × 10⁻⁸ M. Probe 28 should be used in the pH range of 3–9, because the fluorescence induced by Cd²⁺ is stronger in high acidic environment, but it cannot be induced in alkaline environment due to the formation of Cd(OH)₂.

In the same year, W. Su et al. designed probe 29 (Fig. 10) by linking rhodamine 6G hydrazide with N-methylisatin via an imine linkage.⁸ In EtOH/H₂O solution (9/1, 10 mmol HEPES buffer, pH = 7.2), probe 29 alone is non-fluorescent when excited at 500 nm. Upon the addition of Pb²⁺, Hg²⁺, Cd²⁺, the probe 29 generated a yellowish-green fluorescence response to Cd²⁺, while an orange fluorescence responses to Pb²⁺ and Hg²⁺. And the fluorescence intensity at 560/560/552 nm had a linear relationship with the concentration of Pb²⁺/Hg²⁺/Cd²⁺. The detection limits of probe 29 for Pb²⁺, Hg²⁺ and Cd²⁺ were 1.6 × 10⁻⁸, 1.2 × 10⁻⁸ and 4.7 × 10⁻⁸ mol L⁻¹, respectively. When EDTA was added to these complexes, the fluorescence response induced by Hg²⁺ and Cd²⁺ was reversible, while the fluorescence response induced by Pb²⁺ was irreversible. Therefore, probe 29 can be used as a multifunctional probe for detecting Pb²⁺, Hg²⁺ and Cd²⁺. Because the coexistence of Cd²⁺ with Pb²⁺ or Hg²⁺ can enhance the fluorescence of Cd²⁺-induced, and the coexistence of Cd²⁺ with Cu²⁺ or Ni²⁺ can partially quench the fluorescence intensity of Cd²⁺-induced, the selectivity and practicability of the probes 29 for Cd²⁺ are not enough.

In 2018, M. Ghosh and colleagues reported the probe 30 (Fig. 10) for trace-level detection and discrimination of Al³⁺, Zn²⁺, Cd²⁺, Hg²⁺ in a ratiometric sensing mechanism involving PET–CHEF–FRET processes.²³ In 20 mM HEPES-buffered MeOH/H₂O (4/1, v/v, pH = 7.4) solution, probe 30 interacts with Cd²⁺ through phenolic hydroxyl group and “N” in C=N, “O” in alkane proton field to form 1 : 1 complex with Cd²⁺ at a low Cd²⁺ concentration and 1 : 2 at a high Cd²⁺ concentration. When excited at 306 nm, the fluorescence emission peak of probe 30 blue-shifted from 397 nm to 395 nm, the fluorescence intensity increased by 36 times, and the detection limit of probe 30 for Cd²⁺ was 6.7 × 10⁻⁹ M. The association constant of the probe for Cd²⁺ is 1.35 × 10⁵ M⁻¹, and its affinity for Cd²⁺ is higher than that for Zn²⁺. It allows easy replacement of Zn²⁺ from the 30-Zn adduct to form a more stable 30-Cd adduct. Further, the simultaneous presence of Al³⁺ and Hg²⁺ can use KI to mask Hg²⁺, while KI does not interfere with the emission profile of probe 30. Probe 30 can identify low concentrations of Al³⁺ (pink), Zn²⁺ (green), Cd²⁺ (sky blue), Hg²⁺ (intense bloodred) according to different fluorescent signals without interference from other common ions, and can be used for imaging Zn²⁺, Cd²⁺, Hg²⁺ in squamous epithelial cells under a fluorescence microscope.

In 2019, a multi-signaling optical probe 31 (Fig. 11) reported by M. Banerjee's research group for rapid detection and discrimination of Zn²⁺, Cd²⁺ and Hg²⁺ at nano-molar level.³¹ In presence of Zn²⁺, Cd²⁺ and Hg²⁺, it emits deep red for Hg²⁺, green for Zn²⁺ and blue for Cd²⁺ upon UV light irradiation. Interestingly, Hg²⁺ shows intense pink in bare eye. In HEPES-buffered aqueous methanol (MeOH/H₂O, 4/1, v/v, pH = 7.4), the combination of probe 31 and Cd²⁺ in a stoichiometric ratio of 1 : 2 makes the fluorescence emission (the excitation wavelength at 366 nm) peak blue-shift from 440 nm to 437 nm, and the blue fluorescence emission intensity is increased by 31 times. The binding constant of probe 31 for Cd²⁺ is 6.5 × 10⁵ M⁻¹, and the detection limit is 9.6 × 10⁻⁹ M. Probe 31 can be used to detect Hg²⁺, Cd²⁺, Zn²⁺ at the nanomolar level, although the binding ability is sequentially weakened, KI can be used to mask Hg²⁺ interference, Na₂S can be used to mask Cd²⁺ interference.

Fig. 11 Proposed binding mode of probe 31 with Cd²⁺.

The spectroscopic and analytical parameters rhodamine-based sensors are summarized in Table 4. Probes 27, 28 and 29 have large emission wavelengths because their molecular structures have a large degree of conjugation, which makes the fluorescence emission peak appear at a large wavelength. Based on this mechanism, we can develop sensors with larger emission wavelength for environmental detection and biological imaging. In recent five years, rhodamine-based Cd²⁺ fluorescent probes have good sensitivity, but their application is still disturbed by some ions. On the one hand, the probe responds to other analytes, on the other hand, the complex between the probe and Cd²⁺ is disturbed by other analytes. In the future development, it is necessary to optimize the ligands that recognize Cd²⁺, so that the probe and Cd²⁺ have specific recognition ability and stable binding ability. At the same time, improving the water solubility and practicability of rhodamine-based probes is still the focus of future research.

Table 4 Spectroscopic and analytical parameters of the rhodamine-based Cd²⁺ fluorescent probes

Probe	Solvent	Excitation wavelength (nm)	Emission wavelength (nm) λem0–λem	Detection limit (nM)	Association constant (M^−1)	Interfering ion(s)	Ref.
27	CH3CN–H2O (7 : 3)	530	553	18.5	2.33 × 10^5	—	52
28	CH3CN–H2O (2 : 8)	308	480–590	10.25	4.2524 × 10^4	S^2−	32
29	EtOH–H2O (9 : 1)	500	560	47	—	Pb^2+, Hg^2+, Cu^2+, Ni^2+	8
30	MeOH–H2O (4 : 1)	306	397–395	6.7	1.35 × 10^5	Al^3+, Hg^2+	23
31	MeOH–H2O (4 : 1)	366	440–437	9.6	6.5 × 10^5	Hg^2+, Zn^2+	31

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6. Dansyl based Cd²⁺ fluorescent sensor

Dansyl group is fluorescent group with excellent performance as an electron acceptor, so it is used in the design of fluorescence sensors.⁵³ Biological studies have shown that metallothionein is a short peptide rich in cysteine, which has high affinity for a variety of heavy metals. Cd²⁺ and Cu²⁺ ions are the main metal binding with metallothionein.⁵⁴ Therefore, in the design of these probes' structure, P. Wang and co-workers introduced cysteine as the recognition group of Cd²⁺, connected the recognition group and dansyl fluorophore through different spacer groups, and synthesized a series of highly selective probes 32–36 (Fig. 12) for Cd²⁺ by solid-phase peptide synthesis (SPPS) technology.^53,55–57 They described these Cd²⁺ fluorescent turn-on probes. These probes can be used to monitor Cd²⁺ in the environment and in living (HK2,^53,56,57 HeLa,^53–55 LNCaP⁵⁷) cells. In general, they have good water solubility, selectivity, biocompatibility, and reversibility (EDTA,⁵⁵ cysteine⁵³), and can be used for Cd²⁺ detection or bioimaging within some specific environments (example: Cd²⁺ concentration and pH).

Fig. 12 The probes structure reported by P. Wang's team and proposed binding mode with Cd²⁺.

In HEPES buffer (10 mM, pH = 7.4), under 365 nm UV lamp, probe 32 emits weak yellow fluorescence due to electron transfer from sulfhydryl group in Cys to dansyl group.⁵⁵ Binding to Cd²⁺ causes the PET process of probe 32 to be blocked, to enhance fluorescence intensity and to change from yellow to green fluorescence. Upon excitation at 330 ± 10 nm, the fluorescence intensity, quantum yield and lifetime were increased (4 times, from 0.0857 to 0.1769, from 9.32 ns to 17.36 ns). Probe 32 has large stokes shift for detecting Cd²⁺, the binding constant is 5.18 × 10¹⁰ M⁻², and the detection limit is 45 nM. Probe 32 remains stable in 2–10 pH solution and has the strongest fluorescence signal induced by Cd²⁺ in 7–12 pH solution. In addition, the probe 32 can realize the reversible detection for Cd²⁺ by EDTA.

The probe 33 was synthesized by imitating the binding site of protein, in which tryptophan (energy donor, λ_ex = 290 nm, λ_em = 360 nm) and dansyl (energy acceptor, λ_ex = 330 nm, λ_em = 545 nm) are fluorophores and cysteine (recognition) is ionophore.⁵⁴ In the HEPES buffer (10 mM, pH = 7.4) solutions, the probe 33 can detect Cd²⁺ through two different excitation wavelengths. When excited at 290 ± 10 nm, probe 33 combines with Cd²⁺ to meet together the side chains of tryptophan and dansyl group to realize the FRET process, and the fluorescence emission increases at 510 nm and decreases at 360 nm. When excited at 330 nm, the probe 33 binds to Cd²⁺ to increase the fluorescence emission intensity at 545 nm by 5 times (CHEF) and the lifetime from 8.32 ns and 16.33 ns. Under the 365 nm ultraviolet lamp, the solution of the probe 33 is added with Cd²⁺ to emit strong green fluorescence, and other metal ions are added to emit light brown weak fluorescence. The probe 33 is combined with Cd²⁺ at a stoichiometric ratio of 2 : 1, the binding constant is 9.12 × 10¹⁰ M⁻², and the detection limit is less than 27.5 nM. Further, the emission intensity of the complex remains almost unchanged at pH 7–9.

The dansyl-tetrapeptide fluorescent sensor 34 can produce a continuous “off–on–off” fluorescence response to Cd²⁺ and cysteine.⁵³ In the HEPES buffer solution (10.0 mM, pH = 7.4), upon excitation at 330 ± 10 nm, the sensor 34 through oxygen atoms of Glu and sulfydryl of Cys binds to Cd²⁺ at 2 : 1, and the fluorescence changes from yellow to green. The fluorescence emission peak blue-shifts by 30 nm, and the fluorescence intensity increases. Adding Cys to the complex of sensor 34 and Cd²⁺ can restore the fluorescence signal of sensor 34. The detection limits for Cd²⁺ is 93 nM or for Cys is 35 nM, and lower than EPA or WHO guidelines.

Sensor 35 based on the conjugated dansyl group and dipeptide.⁵⁶ In HEPES buffer solutions (20.0 mM, pH = 7.4), under 365 nm ultraviolet light, the combination of sensor 35 and Cd²⁺ prevents the electron transfer from sulfhydryl group in Cys to the dansyl group (PET). Accordingly, the sensor 35 turns on green fluorescence. When excited at 330 nm, the fluorescence intensity at 545 nm is linearly related to the Cd²⁺ of 0–1.40 μM. The association constant and detection limit for Cd²⁺ are 1.30 × 10⁹ M⁻² and 13.8 nM, respectively. Sensor 35 can be used to detect Cd²⁺ with a pH range of 7.0–10.0. Due to the paramagnetism of Cu²⁺, fluorescence of the complex of sensor 35 and Cd²⁺ is partially weakened, but there is no interference from other metal ions.

In 2019, they described probe 36 based on dansyl-appended dipeptide (Gly–Cys–NH₂).⁵⁷ In 20.0 mM HEPES buffer solution at pH 7.4, the addition of Cd²⁺ led to the fluorescence enhancement of the probe, and the formation of dansulfonyl dimer shifted the fluorescence emission peak from 560 nm to 515 nm (monomer-excimer mechanism). The addition of Cu²⁺ led to the fluorescence quenching of the probe. Under 365 nm UV light, the fluorescence of probe 36 was bright green and the emission was significantly enhanced with the addition of Cd²⁺, while the fluorescence of the probe was dark brown and the emission was significantly quenched with the addition of Cu²⁺. The binding constants of probe 36 to Cd²⁺ and Cu²⁺ are 1.74 × 10⁸ M⁻² and 6.96 × 10⁷ M⁻², respectively. The detection limits for Cd²⁺ and Cu²⁺ are 14.5 nM and 26.3 nM, respectively. The probe with pH 7–12 can stably recognize Cd²⁺ (pH 7.0–12.0) and Cu²⁺ (pH 2.0–12.0) in 20 s, and is not interfered by other metal ions and anions. In addition, probe 36 has hypotoxicity, membrane permeability and low toxicity, which can be used to detect Cd²⁺ and Cu²⁺ in living LNCaP cells.

The spectroscopic and analytical parameters danyl-based fluorescent probes are shown in Table 5. The researchers took the sulfhydryl group in cysteine as the recognition site of Cd²⁺, and designed and synthesized these probes by changing the type, number and position of linkers, so as to improve the sensitivity, selectivity and anti-interference of theses probes. Moreover, the researchers use amino acids as recognition groups and linkers, which has low toxicity for biological monitoring. These sensors have good water solubility, selectivity, sensitivity and long emission wavelength. They can be used not only for environmental monitoring, but also for cell imaging.

Table 5 Spectroscopic and analytical parameters of the dansyl-based Cd²⁺ fluorescent probes

Probe	Solvent	Excitation wavelength (nm)	Emission wavelength (nm) λem0–λem	Detection limit (nM)	Association constant	Interfering ion(s)	Ref.
32	H2O	330	550	45	5.18 × 10^10 M^−2	—	55
33	H2O	330	545	27.5	9.12 × 10^10 M^−2	—	54
34	H2O	330	About 570–540	93	—	—	53
35	H2O	330	545	13.8	1.30 × 10^9 M^−2	Cu^2+	56
36	H2O	365	560–515	14.5	1.74 × 10^8 M^−2	Cu^2+	57

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7. Diarylethylene based Cd²⁺ fluorescent sensor

Diarylethylene is composed of heteroaromatic ring groups connected on both sides of the vinyl group. The distance between the two aryl groups is relatively close, which can form two isomers of ring opening and ring closure. As a good photochromic dye, diarylethylene has the advantages of excellent photochromic performance, thermal stability, fatigue resistance, feasible synthesis, high photoisomerization quantum yields, and convenient functionalization.^21,22,28 Therefore, it is often used as a fluorophore part in the design of probes. Diarylethene fluorescent probes have two states: ring-open and ring-closed, which can be converted under ultraviolet light (from ring-open to ring-closed) and visible light. Since the ring-open and ring-closed positions of all diarylethene probes are the same, and the state of the ring does not affect the binding site with Cd²⁺. Therefore, the probe 37 is taken as an example to show the ring-open and ring-closed state of diarylethylene.

In 2017, D. B. Zhang's research group synthesized probe 37 (Fig. 13), which can switch between ring-open and ring-closed under the light irradiation at 297 nm and >500 nm.²¹ In acetonitrile solution, the addition of Cd²⁺/Zn²⁺ changed the probe 37 solution from colourless to light-yellow/dark-yellow. Upon excitation at 402 nm, the fluorescence emission red-shifted from 519 nm to 559 nm/608 nm, and the fluorescence changed from dark to bright-yellow/orange. Under 297 nm light irradiation, the ring-open complex of Cd²⁺ and probe 37 changed into a ring-closed complex, and the fluorescence intensity of the complex was quenched by 8.7%/19%. The fluorescence of the ring-open complex was recovered under the irradiation light >500 nm. The detection limits of probe 37 for Cd²⁺ is 3.2 × 10⁻⁷ M and for Zn²⁺ is 2.88 × 10⁻⁷ M, and the association constants (log K_a) for Cd²⁺ is 3.81 and for Zn²⁺ is 3.51. Probe 37 is selective and reversible (EDTA) for the detection of Zn²⁺ and Cd²⁺. The water solubility of the probe 37 is poor, and its coordination ability to Cd²⁺ is stronger than that to Zn²⁺, so Cd²⁺ can replace Zn²⁺ binding probe 37. A logic circuit was constructed with stimuli of UV/vis lights, Cd²⁺ and EDTA as inputs and fluorescence intensity at 559 nm as an output. In addition, because probe 37 contains a hydrazinobenzothiazole group, it can react with acids and bases, and the present of OH⁻ is detected in the near infrared region.

Fig. 13 Proposed binding mode of probes 37 to 39 with Cd²⁺.

In the same year, X. X. Zhang et al. reported that sensor 38 (Fig. 13).²² In THF solution, under 360 nm light, adding Cd²⁺, Ag⁺, Zn²⁺ increases the fluorescence of sensor 38, but only Cd²⁺ makes the fluorescence emission peak redshift from 439 nm to 541 nm and the fluorescence changes from dark to bright-green. 0–3.0 equiv. of Cd²⁺ has a linear relationship with the fluorescence intensity ratio (I_{541 nm}/I_{439 nm}). The ring-open and ring-closed states of sensor 38 can be converted by 313 nm and >450 nm light. Under 313 nm irradiation, the solution changes from pale yellow to magenta, and the fluorescence intensity reduces by about 87% compared with the open-ring complex. The sensor 38 detects Cd²⁺ with selectivity and reversibility, the detection limit for Cd²⁺ is 1.97 × 10⁻⁷ mol L⁻¹, and the binding constant (K_a) to Cd²⁺ is 8.60 × 10³ L mol⁻¹. A logic circuit was fabricated with four inputs of the combinational stimuli of UV/vis and Cd²⁺/EDTA, and one output of fluorescence intensity at 541 nm.

In 2018, S. Guo and colleagues designed probe 39 (Fig. 13) and studied its recognition of Cd²⁺ in 392 nm excitation and acetonitrile solution.⁵⁸ When Cd²⁺ was added, the solution of the open-ring isomer of probe 39 changed from colorless to yellow, the fluorescence emission peak red-shifted from 507 nm to 633 nm, the fluorescence changed from dark cyan to golden yellow, and the fluorescence intensity increased by 24.9 fold. After illuminating at 297 nm, a complex of the closed-ring isomer of probe 39 and Cd²⁺ was formed, the solution changed from yellow to plum and appeared absorption band at 547 nm, the fluorescence changed from golden yellow to fawn brown. Cd²⁺ was directly added to probe 39 solution of closed-ring isomer, the color changed from purple to plum, and the fluorescence changed from dark to fawn brown. Due to the incomplete cyclization and the formation of parallel conformational isomers, the compound fluorescence of the ring-closed and Cd²⁺ was reduced by 34%. In addition, HSO₃⁻ made the probe 39 emit bright cyan fluorescence and increase the intensity 135 times. The probe 39 can recognize Cd²⁺ and HSO₃⁻ with high selectivity in the acetonitrile solution, and the binding with Cd²⁺ is irreversible.

In 2018, Z. Wang et al. synthesized the sensors 40 and 41 (Fig. 14), the conversion between their open-ring and closed-ring isomers can be realized by irradiation at 297 nm and >500 nm.^18,28 The properties of the two probes were studied in methanol solution and 350 nm light excitation. Because EDTA can make these probes bind to Cd²⁺ reversibly, the author used Cd²⁺/EDTA and UV/vis as the input stimulus and the fluorescence intensity (sensors 40 and 41 were recorded at 476 nm and 480 nm, respectively) of probes in the presence of Cd²⁺ as the output to construct the logic circuit.

Fig. 14 The probes structure reported by Z. Wang's team and proposed binding mode with Cd²⁺.

Sensor 40 has a dark purple weak fluorescence at 435 nm.²⁸ Adding Cd²⁺, the C=N isomerization of the sensor was inhibited, the fluorescence changed from blue to bright blue, and maximum emission peak red-shifted from 435 nm to 476 nm. Under 297 nm, the colorless open-ring isomer transforms into pink closed-ring isomer, the emission intensity at 435 nm reduces about 73% and the fluorescence changes from blue to dark, and adding Cd²⁺ makes the fluorescence change from dark to dark blue. Sensor 40 can detect Cd²⁺ as low as 2.52 × 10⁻⁷ mol L⁻¹, and the binding constant (K_a) is 4.3 × 10³ L mol⁻¹. Sensor 40 with poor water solubility, and the fluorescence of Cd²⁺-induced can be completely quenched by Cu²⁺.

Sensor 41 chelates Cd²⁺/Zn²⁺, which inhibits the C=N isomerization, increases (51.1/17.7 fold) the fluorescence, and changes from dark to light-blue/green fluorescence.¹⁸ In addition, the emission peak at 451 nm red shifts to 480 nm/500 nm, and the fluorescence intensity at 480 nm/500 nm is linearly related to the concentration of Cd²⁺/Zn²⁺. The complexing constants of open-ring isomer for Cd²⁺ is 5.95 × 10⁴ L mol⁻¹ and for Zn²⁺ is 4.9 × 10⁴ L mol⁻¹, and the detection limits for Cd²⁺ is 8.69 × 10⁻⁹ mol L⁻¹ and for Zn²⁺ is 3.67 × 10⁻⁸ mol L⁻¹. Under the irradiation at 297 nm, the open-ring isomer became a weaker fluorescent closed-ring isomer and the solution change from colorless to purple. Adding Cd²⁺/Zn²⁺ made it change from light dark fluorescence to dark-blue/dark-green, the fluorescence intensity increase by 28.5/14.5 times, and the emission peak red-shift 29 nm/49 nm. Probe 41 can selectively and reversibly distinguish Cd²⁺ and Zn²⁺.

In 2019, H. L. Liu et al. designed a fluorescence enhanced probe 42 (Fig. 15) based on C=N isomerism suppression and CHEF mechanism to identify Cd²⁺.²⁵ They tested the probe in methanol solution and at 350 nm excitation wavelength. The open-ring isomer of probe 42 combined with Cd²⁺ increased the fluorescence emission intensity by 106 times, red-shifted the emission peak from 452 nm to 506 nm, and changed the fluorescence from dark to cyan. The association constant between probe and Cd²⁺ was 3.14 × 10⁴ L mol⁻¹, and the detection limit for Cd²⁺ was 1.89 × 10⁻⁷ mol L⁻¹. The closed-ring isomer of probe 42 changed from non-fluorescent to dark-cyan fluorescence after binding to Cd²⁺, and the emission peak red-shifted from 452 nm to 511 nm. Due to the formation of the closed-ring isomer and the FRET process from 2-aminoisoindole-1,3-dione to diethylene unit, its fluorescence intensity is 63% weaker than that of the complex of Cd²⁺ and the open-ring isomer. With 297 nm and >500 nm light irradiations, the conversion between the open-ring isomer and the closed-ring isomer of probe 42 can be realized. Adding Cu²⁺ made the open-ring isomer solution of probe 42 changed from colorless to yellow and the closed-ring isomer solution from purple to green, and it can be used for naked eye colorimetric detection of Cu²⁺ with the detection limit 4.12 × 10⁻⁸ mol L⁻¹. As a multifunctional fluorescent probe, the probe 42 can selectively and reversibly bind and accurately detect Cd²⁺ and Cu²⁺, but Cu²⁺ can obviously quench the fluorescence intensity caused by Cd²⁺.

Fig. 15 Proposed binding mode of probes 42 and 43 with Cd²⁺.

J. F. Lv's research group designed and synthesized a multifunctional fluorescent sensor 43 (Fig. 15) based on diarylethene containing pyrene unit.⁵⁹ In acetonitrile solution, adding Cd²⁺ or Zn²⁺ makes the absorption peak of the sensor red shift and the solution color change. Under the excitation at 440 nm, the presence of Cd²⁺ or Zn²⁺ not only changes the fluorescence color, but also increases the fluorescence intensity (due to the C=N isomerization blocking and CHEF effect), and the fluorescence emission intensity is linearly related to a certain range of ion concentration. The binding constants of the open-loop sensor with Cd²⁺ or Zn²⁺ are 5.8 × 10⁴ L mol⁻¹ or 6.0 × 10⁴ L mol⁻¹, respectively. The detection limits for Cd²⁺ or Zn²⁺ are 1.85 × 10⁻⁹ mol L⁻¹ or 7.68 × 10⁻⁹ mol L⁻¹, respectively. Sensor 43 can be used for the detection of Cd²⁺ and Zn²⁺ in actual water samples, and processed into test pieces for on-site analysis and testing. However, when the sensor is used for the fluorescence detection of Cd²⁺ (affected by Mg²⁺, Zn²⁺, Sn²⁺, Co²⁺, Cu²⁺, Ni²⁺) and Zn²⁺ (affected by Cd²⁺, Hg²⁺, Cu²⁺, Ni²⁺), it is easy to be interfered by a variety of cations. Moreover, because the absorption peak, emission peak and color change caused by Cd²⁺ or Zn²⁺ are relatively close, sensor 43 can't distinguish them effectively by naked eye.

The spectroscopic and analytical parameters of the diarylethene-based Cd²⁺ fluorescence sensor are shown in Table 6. Different from fluorophores such as quinoline, coumarin and benzothiazole, diarylethylene is only used as fluorophore (signal reporter) in probe design. The diarylethylene-based probes can realize the ring-open and ring-closed transition through ultraviolet and visible light, and its ring-open state recognition Cd²⁺ can produce greater fluorescence changes. Some sensors diarylethylene-based have relatively large emission wavelengths. However, many sensors either respond to ions other than Cd²⁺, or are disturbed by other ions when detecting Cd²⁺, and have low selectivity and anti-interference ability. Moreover, their water solubility and detection limit need to be improved.

Table 6 Spectroscopic and analytical parameters of the diarylethylene-based Cd²⁺ fluorescent probes

Probe	Solvent	Excitation wavelength (nm)	Emission wavelength (nm) λem0–λem	Detection limit (nM)	Association constant (Ka)	Interfering ion(s)	Ref.
37	CH3CN	402	519–559	320	log Ka = 3.81	Zn^2+	13
38	THF	360	439–541	197	8.60 × 10^3 M^−1	Ag^+, Cu^2+, Pb^2+	22
39	CH3CN	392	507–633	—	—	Cu^2+	58
40	MeOH	350	435–476	252	4.3 × 10^3 M^−1	Cu^2+	28
41	MeOH	350	451–480	8.69	5.95 × 10^4 M^−1	Zn^2+	18
42	MeOH	350	452–506	189	3.14 × 10^4 M^−1	Cu^2+	25
43	CH3CN	440	648	1.85	5.8 × 10^4 M^−1	Mg^2+, Zn^2+, Sn^2+, Co^2+, Cu^2+, Ni^2+	59

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8. Other small organic molecules based Cd²⁺ sensor

In 2017, S. Chithiraikumar's research group synthesized sensor 44 (Fig. 16), which can recognize Cd²⁺ through absorption and fluorescence spectra.¹² In acetonitrile/HEPES buffer medium (5 mM, pH = 7.3, 1 : 5, v/v), the sensor 44 combines with Cd²⁺ to red-shift its maximum absorption from 391 nm to 418 nm, and the absorbance of probe at 418 nm to increase with the concentration of Cd²⁺, and the association constant for Cd²⁺ was 5.39 × 10⁵ M⁻¹ (±1.0%). Under the excitation at 400 nm, the sensor 44 binds to Cd²⁺ with an association constant of 3.16 × 10⁵ M⁻¹, which makes the fluorescence emission peak redshift from 558 nm to 561 nm, and the fluorescence emission intensity and quantum yield increase. The sensor 44 binds to Cd²⁺ in a pH system of 4.2 to 8.1, rapidly increases fluorescence within 0.3 min and without interference from various ions to monitor Cd²⁺ as low as 10.21 nM. The sensor 44 has selectivity, sensitivity, reversibility (EDTA) and bio-imaging (HeLa cells) for detecting Cd²⁺, the fluorescence response is enhanced when the temperature increases within 25–45 °C, and the fluorescence is quenched when the aqueous buffer is 80–100%.

Fig. 16 Proposed binding mode of probes 44 to 47 with Cd²⁺.

D. D. Cheng and colleagues reported on the ICT probe 45 (Fig. 16) with tetramethyl substituted bis(difluoroboron)-1,2-bis[(1H-pyrrol-2-yl)methylene]hydrazine (Me₄BOPHY) as a fluorophore and N,N-bis(pyridin-2-ylmethyl)benzenamine (BPA) as an electron donor moiety.¹⁹ Chelating Cd²⁺ reduces the electron-donating ability of BPA, thus quenching ICT transition and initiating π–π transition of the fluorophore, resulting in blue shift of the absorption and emission of the probe. In acetonitrile solution, Cd²⁺ made the absorption peak of probe 45 blue-shift from 550 nm to 475 nm. Under the excitation at 410 ± 10 nm, the fluorescence emission peak blue-shifted from 675 nm to 570 nm, and the solution changed from red to bright yellow. When the excitation wavelength at 495 nm, the fluorescence intensity ratio F_{570 nm}/F_{730 nm} reached a stable value within 1 min and had a functional relationship with the concentration of Cd²⁺, which could be used to quantitatively detect Cd²⁺. The probe 45 also has a fluorescence response to Zn²⁺, but still emits red fluorescence. Therefore, the probe 45 can detect Cd²⁺ as low as 6.9 nM or 0.77 ppb with selectivity, sensitivity, and anti-interference, which is far below the safety value (3 ppb) set for drinking water by WHO.

W. B. Huang et al. synthesized probe 46 (Fig. 16), in which DPA was used as the recognition group of Cd²⁺, and two sulfonic groups were introduced into the porphyrin fluorophore to enhance the water solubility.⁶⁰ The HEPES buffer (20 mM, pH = 7.4) and the excitation at 418 nm were selected to study. Probe 46 and Cd²⁺ combined with a stoichiometric ratio of 1 : 2, weakened the conjugation between DPA and porphyrin (ICT), so that the fluorescence emission blue-shifted from 653 nm to 611 nm, and the fluorescence intensity ratio of F_{611 nm}/F_{653 nm} was linearly related to the Cd²⁺ concentration of 0–2.5 μM. The dissociation constant was 31.2 ± 5.2 μM, and the detection limit for Cd²⁺ was 3.2 × 10⁻⁸ M. Hg²⁺ and Cu²⁺ showed moderately quenching of fluorescence signal at 653 nm, but there was no enhancement of fluorescence signal at 611 nm. In an environment with no Hg²⁺ or Cu²⁺ and a pH of 6.5–10, the probe 46 can quantitatively and reversibly (EDTA) detect Cd²⁺. In addition, the probe has low cytotoxicity and can be used for bio-imaging in living cells.

J. Q. Sun's research group designed a multi-responsive squaraine-based sensor 47 (Fig. 16).¹⁷ In ethanol/H₂O (9 : 1) solution (10 mM HEPES buffer at pH 7.0), when excited at 470 nm, sensor 47 shows weak fluorescence at 545 nm due to ESIPT and C=N isomerization. Adding Cd²⁺/Al³⁺/Zn²⁺ made the sensor 47 increase the fluorescence intensity by 39/31/53 times, and shift the emission peak from 545 nm to 537 nm/550 nm/542 nm, and emit bright-green/yellow/yellow-green fluorescence under 365 nm irradiation. The stability of the combination of sensor 47 with Al³⁺, Zn²⁺, Cd²⁺ decreased in turn. Sensor 47 can detect Cd²⁺ as low as 5.76 × 10⁻⁸ M at pH 5–10, and all the other metal ions except Zn²⁺ can decrease the fluorescence enhancement caused by Cd²⁺ to a different extent. Moreover, the sensor 47 can chelate Al³⁺ from Aβ1-42-Al complex, indicating its potential value in the treatment of Alzheimer's disease.

N. A. Bumagina et al. described probe 48 (Fig. 17), which was excited at 495 nm and studied its ion recognition in a solution of propanol-1/cyclohexane (1 : 30).⁶¹ Adding Cd²⁺/Hg²⁺ made the fluorescence change from yellow-orange to orange-green, and the maximum emission peak of probe 48 red-shift from 518 nm to 538 nm/539 nm. When the molar ratio of Cd²⁺ concentration to probe concentration is less than 5, the fluorescence intensity increases linearly with the increase of Cd²⁺ concentration. When the molar ratio of Cd²⁺ concentration to probe concentration is greater than 5, the ratio of I_{538 nm}/I_{518 nm} increases to 150 times and remains unchanged. Similarly, when the molar ratio of Hg²⁺ concentration to probe concentration is greater than 4, I_{539 nm}/I_{518 nm} increases by 40 times and remains unchanged. Probe 48 complexes Cd²⁺/Hg²⁺ with a stoichiometric ratio of 2 : 2, and the detection limit is 2 × 10⁻⁹ M/1.7 × 10⁻⁸ M, which can effectively detect Hg²⁺, but Co²⁺, Cu²⁺, Zn²⁺, Hg²⁺ interfere with the detection of Cd²⁺.

Fig. 17 Proposed binding mode of probes 48 and 49 with Cd²⁺.

The sensor 49 (Fig. 17) synthesized by V. Tekuri et al. has good selectivity, precision and accuracy for the detection of Cd²⁺, and can be used for colorimetric detection of Cd²⁺ in environmental samples.⁶² When Cd²⁺ was added to the DMF solution of the new heterocyclic thiophene-2-carboxylic acid hydrazide based sensor 49, the solution changed from colorless to yellow, the maximum absorption red-shifted from 320 nm to 425 nm, and the absorption intensity had a linear relationship with the concentration of Cd²⁺ from 5.0 to 30 μM. The association constant (K_a) was 3.7 × 10⁴ M⁻¹, and the detection limit was 2.0 × 10⁻⁷ M.

Y. Tang's research group synthesized probe 50 (Fig. 18) by a click reaction of dialkyne and indole azide precursors.⁶³ In ethanol–ethyl acetate (7 : 3, v/v) solution, under 283 nm, adding Cd²⁺/Fe³⁺ made the maximum fluorescence emission of probe 50 at 510 nm be quenched. But only the concentration of Cd²⁺ from 0 to 7.226 × 10⁻⁵ M was linearly related to the fluorescence intensity, which can be used to quantitatively detect Cd²⁺, and the detect limit for Cd²⁺ is 2.69 μM. The fluorescence quenching of probe 50 by Cd²⁺ reached equilibrium within 30 minutes, and it was obviously quenched at 20–50 °C, and reached the best quenching at 70 °C.

Fig. 18 The structure of probes 50 to 53.

The biphenyl substituted piperidine-4-one sensor 51 (Fig. 18) in aqueous solution containing 1% CH₃CN, upon irradiation at 315 nm, the fluorescence at 433 nm was increased by 3.6 times with the addition of Cd²⁺.⁶⁴ And the maximum fluorescence intensity of sensor 51 has a linear relationship with the Cd²⁺ concentration of 0 to 110 equivalents, which can quantitatively detect Cd²⁺. The binding constant and the detection limit for Cd²⁺ were 3862.872 M⁻¹ and 1 × 10⁻⁷ mol L⁻¹, respectively. The sensor 51 reported by S. Poomalai et al. can selectively and sensitively respond to Cd²⁺ in the pH range of 4.5 to 8.0, and can be incorporated into the prepared polysulfone membranes to effectively detect and remove Cd²⁺ in real samples.⁶⁴

The first report of probe 52 (Fig. 18) was reported by W. H. Hsieh et al. in 2012 as a probe for Zn²⁺ recognition.⁶⁵ In 2017, P. Kaur et al. supplemented the ion recognition of this probe.⁶⁶ In methanol solution, the solution immediately changed from yellow to colorless when probe 52 and Zn²⁺/Cd²⁺ were combined at a ratio of 1 : 2. The probe 52 chelated Zn²⁺/Cd²⁺ to inhibit C=N isomerization. Upon excitation at 308 nm, adding Zn²⁺ and Cd²⁺ made probe 52 turn on fluorescence at 450 nm, the detection limits for Zn²⁺ was 86 nM and for Cd²⁺ was 50 nM, and the binding constant with Zn²⁺ was 4.60 and for Cd²⁺ was 4.87. If there is IO₄⁻ in the detection system, there is no turn-on fluorescence response caused by Zn²⁺/Cd²⁺. In addition, the probe 52 can distinguish Fe³⁺ (light green) and Fe²⁺ (purple) by naked eye.

In 2017, the study team of B. K. Momidi designed the multi-signaling thiocarbohydrazide based colorimetric sensor 53a and 53b (Fig. 18).⁶⁷ 53a can specifically bind to Cd²⁺, 53b can multi-signal response to Hg²⁺, Cu²⁺, Cd²⁺, Pb²⁺ and the selectivity is sequentially weakened, and both sensors have good linear range 0–10⁻⁶ M, which can quantitatively detect metal ions in aqueous medium. In DMSO solution, 53a has no response to other metal ions, but when combined with Cd²⁺ at a stoichiometric ratio of 1 : 1 in DMSO solution, the solution changes from colorless to pale pink, and the maximum absorption band red-shifts from 395 nm to 579 nm. The detection limit of 53a for Cd²⁺ is 3.22 μM. In DMF solution, 53b has an absorption band at 354 nm. When 53b reacts with Cd²⁺, Hg²⁺, Cu²⁺ and Pb²⁺ at a stoichiometric ratio of 1 : 1, the solution changes from colorless to brownish-red, pink, yellow and orange, and the absorption peaks shows red shifted by 146 nm, 96 nm, 66 nm and 96 nm, respectively. The detection limits of 53b for Cd²⁺, Hg²⁺, Cu²⁺ and Pb²⁺ are 0.20 μM, 0.70 μM, 0.20 μM and 0.30 μM, respectively. The multi-sensing ability of 53b is due to the presence of hydroxy functionality of quinoline moiety, which has suitable binding sites (–OH, C=N and N in quinoline ring) to co-ordinate with the metal ions.

V. Kumar's team synthesized a sensor 54 (Fig. 19) with biphenyl as the fluorophore and carboxylic acid group as the binding site.⁶⁸ In CH₃OH solution, under excitation at 320 nm, sensor 54 emits strong fluorescence at 418 nm. Adding Cd²⁺ made the fluorescence emission peak blue-shift of sensor 54 by 25 nm and the fluorescence be quenched. Adding Al³⁺ made the fluorescence emission peak of sensor 54 red-shift 3 nm and fluorescence be increased. Adding S²⁻ made the fluorescence emission peak of sensor 54 blue shift by 7 nm and the fluorescence be quenched. The sensor 54 combines Cd²⁺, Al³⁺, and S²⁻ at a stoichiometric ratio of 1 : 1, and the detection limits for them are 1.02 μM, 0.55 μM, and 16.57 μM, respectively. When Cd²⁺, Al³⁺, and S²⁻ exist at the same condition, the combination of sensor 54 and Al³⁺ is dominant, and K₂HPO₄ can realize the reversibility of sensor 54 to detect Cd²⁺/Al³⁺.

Fig. 19 Proposed binding mode of probes 54 and 55 with Cd²⁺.

Among the probes reported in 2017, in addition to those mentioned above, there is a ferrocenyl Schiff base sensor 55 (Fig. 19) reported by M. Findik's team.⁶⁹ In an acetonitrile solution and an excitation at 378 nm, the sensor 55 bound to Zn²⁺/Cd²⁺ enhanced the weak fluorescence emission at 452 nm by 12.73/4.59 times (due to the C=N isomerization inhibition and CHEF), and the fluorescence intensity can reflect the concentration of Zn²⁺/Cd²⁺. The detection limits for Zn²⁺ was 0.68 μM and for Cd²⁺ was 0.94 μM, and the binding constants for Zn²⁺ was (1.38 ± 0.25) × 10⁶ M⁻¹ and for Cd²⁺ was (7.49 ± 0.18) × 10⁵ M⁻¹. In acetonitrile solution, although the sensor 55 can reversibly (EDTA) detect Zn²⁺ and Cd²⁺, the affinity for Zn²⁺ is stronger than that for Cd²⁺.

In 2018, R. Purkait's research group described a multi-analyte responsive sensor 56 (Fig. 20).¹³ In the DMSO/water HEPES buffer (pH = 7.2; 9 : 1, v/v) and the excitation wavelength at 482 nm, adding Zn²⁺/Cd²⁺ enhanced the ICT effect of the sensor 56, due to the deprotonation of the phenolic-OH of p-cresol moiety may facilitate charge transfer transitions. Adding Zn²⁺/Cd²⁺, the sensor 56 had the maximum emission at 545/560 nm, and it emitted bright-yellow/orange fluorescence under ultraviolet light. The binding of sensor 56 to Zn²⁺ and Cd²⁺ is reversible (EDTA), the binding constants for Zn²⁺ is 2.7 × 10⁴ M⁻¹ and for Cd²⁺ is 0.96 × 10⁴ M⁻¹, and the detection limits for Zn²⁺ is 2.7 × 10⁻⁹ M and for Cd²⁺ is 6.6 × 10⁻⁹ M, which is lower than permissible levels for these ions in drinking water by EPA. In addition, sensor 56 can detect I⁻as low as 5 nM based on ESIPT. When the thin-layer chromatography plates stained with sensor 56 for field detection, Zn²⁺, Cd²⁺, I⁻ change the light red fluorescence to green, orange, and red respectively.

Fig. 20 Proposed binding mode of probes 56 and 57 with Cd²⁺.

In 2018, N. Behera et al. reported that hydrazide probe 57 (Fig. 20).¹⁰ Probe 57 can detect Al³⁺/Zn²⁺/Cd²⁺ as low as 5.7 × 10⁻⁹ M/1.09 × 10⁻⁶ M/1.64 × 10⁻⁶ M in physiological pH and without Cu²⁺, and the complexes of this probe and these metal ions can be used for detecting F⁻ or H₂PO₄⁻. In MeOH–HEPES buffer solution (5 mM, pH = 7.3, 7 : 3 v/v), when excited at 435 nm, probe 57 emitted weak fluorescence at 495 nm due to the PET effect caused by electron transfer from nitrogen atom of imino (–CH=N–) group to conjugated π system of naphthalene fluorophore. Adding Al³⁺/Zn²⁺/Cd²⁺ turned on the fluorescence of probe 57, the fluorescence intensity increased by 95/43/49 times, the emission peak shifted from 495 nm to 485/570/540 nm. But the presence of Cu²⁺ could quench the fluorescence intensity of those complexes about 20–30%. The 2 : 1 complex formed by probe 57 and Al³⁺ can detect F⁻ as low as 2.5 × 10⁻⁶ M by fluorescence quenching, and the 1 : 1 complex formed by probe 57 and Zn²⁺/Cd²⁺ can detect H₂PO₄⁻ as low as 1.43 × 10⁻⁵ M/2.11 × 10⁻⁵ M by fluorescence quenching.

A bis(salamo)-type tetraoxime sensor 58 (Fig. 21) with two N₂O₂ chelating moieties as ionophore.⁵ In Tris–phosphate buffer solution (c = 0.2 M, DMF/H₂O = 9 : 1, v/v, pH = 7.0), the change of absorbance in UV vis test shows that sensor 58 can combine with Cd²⁺, Ni²⁺, Zn²⁺, and Mn²⁺. However, only Cd²⁺ changes from faint yellow to green under 365 nm light. When the excitation wavelength at 323 nm, adding Cd²⁺ made the emission peak of sensor 58 (due to the PET inhibition and CHEF) red-shift from 412 nm to 486 nm, the fluorescence intensity increase from 125 to 712, and the fluorescence quantum yield increase from 2.6% to 13.8%. EDTA enables the sensor 58 to detect Cd²⁺ reversibly and the fluorescence emission at 373 nm can be used as a signal output to establish a molecular logic gate. In an aqueous system with a pH of 3.0 to 11.0 and a temperature of 0 to 90 °C, the sensor 58 synthesized by J. Hao's team can recognize Cd²⁺ with naked eyes (the solution changes from colorless to yellow) and fluorescence, and detect as low as 8.61 × 10⁻⁷ M without being interfered by other metal ions. The binding constant of the sensor to Cd²⁺ is as 4.98 × 10⁴ M⁻¹. In addition, a test strip for the sensor 58 can be made to detect Cd²⁺ in food and the environment.

Fig. 21 Proposed binding mode of probes 58 to 60 with Cd²⁺.

A phenylacetylene based sensor 59 (Fig. 21) reported by Y. Y. Zhang et al. can be used for colorimetric detection of Fe²⁺ and fluorescence proportional detection of Cd²⁺, Zn²⁺.⁷⁰ Adding Zn²⁺/Cd²⁺ to the ethanol/water (1 : 1, v/v) solution of sensor 59 enhanced the electron withdrawing ability of terpyridine and enhanced the ICT effect. Upon excitation at 340 ± 5 nm, the addition of Zn²⁺/Cd²⁺ caused the fluorescence of sensor 59 to change from blue to yellow-green/cyan, the fluorescence emission to decrease at 408 nm and to increase at 498 nm/473 nm. The fluorescence intensity ratios F_{498 nm}/F_{408 nm} and F_{473 nm}/F_{408 nm} were linearly related to the Zn²⁺ and Cd²⁺ concentrations, respectively. The detection limits of sensor 59 for Zn²⁺ and Cd²⁺ are 2.03 × 10⁻⁹ M and 2.62 × 10⁻⁸ M, respectively. However, Fe²⁺ can interfere the fluorescence turn on of the sensor to Zn²⁺ or Cd²⁺. Adding Fe²⁺, it can be observed by naked eyes that the sensor 59 solution changed from colorless to purplish red. In addition, this multifunctional sensor can be used for the detection of these ions in water samples and test strips.

Also in 2018, the M. Formica's research group synthesized probes 60a and 60b (Fig. 21).⁷¹ The probes contain the 2,5-diphenyl[1,3,4]oxadiazole (PPD) fluorophore, methylene spacer and two coordinating units, namely dipicolylamine (DPA) for 60a and 1,4,7-triazacyclononane (TACN) for 60b. In 0.15 mol L⁻¹ NMe₄Cl hydroalcoholic solution (water/ethyl alcohol 50/50 v/v), upon the excitation at 277 nm, Cd²⁺ and 60a only forms a mononuclear complex and emits high fluorescence at 368 nm. Cd²⁺ and 60b forms mononuclear and dinuclear complexes, and forms excimer to emit fluorescence at 474 nm. Probes 60a and 60b complex with Cd²⁺ at 1 : 1, with Zn²⁺ at 1 : 2 in water or hydroalcoholic medium, but the response to Zn²⁺ is not as sensitive as Cd²⁺. Because only the mononuclear complex has a fluorescence response, so the concentration must be higher than that of the analyte when used as a sensor.

T. H. Liu and coworkers synthesized probe 61 (Fig. 22) by covalent combination of terthiophene with 2,2′:6′,2′′-terpyridine.²⁶ In methanol solution, probe 61 forms a 2 : 1 complex with Cd²⁺, which red-shifts the maximum absorption peak from 396 nm to 424 nm. Upon excitation at 395 nm, the fluorescence of the probe 61 changes from green to bright yellow, appears a new emission centering at 572 nm and decreases the original emission around 504 nm. These changes can distinguish the response of this probe to other metal ions (Co²⁺, Cu²⁺, Ni²⁺, Zn²⁺, etc.). Probe 61 can detect Cd²⁺ in a pH environment of 5–8 as low as 1.74 × 10⁻⁷ M, and the binding constant is 2.25 × 10⁵ M⁻¹. Alternating addition of metal ions and EDTA can't achieve the reversibility of the fluorescence changes induced by metal ion. Cu²⁺, Co²⁺ or Zn²⁺ not only blocks the probe 61 from forming a complex with Cd²⁺, but also ensures the quenching of the fluorescence.

Fig. 22 The structure of probes 61 and 62.

In 2019, J. K. Xiong's team reported the sensor 62 (Fig. 22) of a perylene bisimide derivative conjugated with lactose and DPA.¹⁴ In an aqueous solution with a pH of 3.0–9.5, the sensor 62 can highly and selectively bind to Cd²⁺ in a stoichiometric ratio of 2 : 1, and the binding constant is (6.0 ± 0.3) × 10⁴ M⁻¹. Although this sensor can detect Cd²⁺ as low as 5.2 × 10⁻⁷ M, the present of Zn²⁺, Co²⁺ or Cu²⁺ can quench the fluorescence induced by Cd²⁺.

P. Ravichandiran et al. designed probe 63 (Fig. 23) by introducing a furan-2-carboxamide group to a phenoxazine fluorophore and a phenyl sulfonyl chelating site.⁷² In CH₃CN/H₂O (99 : 1, v/v) medium, when excited at 340 nm, probe 63 emitted weak fluorescence due to the C=N isomerization and ICT effects. The combination of probe 63 with Cd²⁺ strengthened the charge transfer between the secondary amine and the amide group (–CONH) of the benzoxazine moiety and enhanced the ICT effect. In addition, the use of polar organic solvent acetonitrile, these result in a 130-fold increase in the fluorescence of probe 63 at 530 nm. The binding constant is 1.36 × 10⁶ M⁻², and the detection limit for Cd²⁺ is 0.60 μM. Probe 63 recognizes Cd²⁺ with selectivity and reversibility (EDTA), and can be used for detecting Cd²⁺ in living cells. In addition, the complex of probe 63 and Cd²⁺ can turn-off the fluorescence to detect CN⁻ with a detection limit of 0.145 μM.

Fig. 23 Proposed binding mode of probes 63 to 65 with Cd²⁺.

The research group of M. Das synthesized a pyridyl based Schiff base sensor 64 (Fig. 23).⁷³ In methanol solution, under excitation wavelength at 290 nm, adding Cd²⁺ and Ni²⁺ quenched the fluorescence of sensor 64. The sensor 64 has maximum absorption at 234 nm and 272 nm, adding Cd²⁺/Ni²⁺ led to the maximum absorption peaks red-shift from 272 nm to 280/281 nm, and the interference of other metal ions and anions was neglected. The binding constants of sensor 64 to Cd²⁺ and Ni²⁺ were 4.29 × 10⁶ L mol⁻¹ and 26.88 × 10⁶ L mol⁻¹ calculated by Yoe Jones method, respectively. The detection limit for Cd²⁺ was 1.4688 μM, and for Ni²⁺ was 3.08 μM.

M. Vera et al. designed sensor 65 (Fig. 23).²⁹ They inserted –CH₂– between the DTPA (diethylenetriaminepentaacetate) and the naphthalene chromophore to make the complex stable and have little impact on the environment when Cd²⁺ was detected by sensor 65. Furthermore, sensor 65 can distinguish Zn²⁺ and Cd²⁺, and without fluorescence quenching caused by paramagnetic effect. In an aqueous solution and the excitation wavelength at 280 nm, sensor 65 has strong naphthalene-monomer emission at 355 nm and excimer emission at 410 nm. The sensor 65 is coordinated with Cd²⁺, so that the fluorescence emission of excimer is increased by 70–100%, while the monomer emission is hardly affected. The sensor 65 is coordinated with Zn²⁺, so that the fluorescence emission of excimer is decreased by 60%, while the monomer emission is increased.

P. Kaur's research group synthesized a 1,2,3-triazole linked fluorescein derivative probe 66 (Fig. 24).³⁰ In an ethanol/water (4 : 1, v/v) solution, probe 66 has high selectivity to Cd²⁺ and Co²⁺. It binds to Cd²⁺ and Co²⁺ through the N, O, N cornered caged of probe 66. Due to the PET effect, probe 66 shows fluorescence turn off for Cd²⁺ and fluorescence turn on for Co²⁺, and the detection limits at 9–10 μM.

Fig. 24 Proposed sensing mode of probes 66 and 67 with Cd²⁺.

Using 2,9-dimethyl-1,10-phenanthroline and o-aminophenol as raw materials, M. X. Huang et al. synthesized a simple, sensitive and practical fluorescence enhanced probe 67 (Fig. 24) for Cd²⁺.⁷⁴ The performance of probe 67 to recognize Cd²⁺ was tested in DMF–water (v/v, 3 : 7) solution, excitation wavelength at 275 nm, and emission wavelength at 377 nm. The addition of Cd²⁺ restricted the PET process (electron transfer from the N of imine to phenanthroline ring) and the C=N isomerization, resulting in charge rearrangement and rigidity enhancement. Therefore, Cd²⁺ led to fluorescence enhancement and fluorescence lifetime reduction of the probe 67. The binding constant between the probe 67 and Cd²⁺ is 3.15 × 10⁵ M⁻¹. The emission intensity of probe 67 at 377 nm has a good linear relationship with Cd²⁺ at 0.025–2.5 μM, and it can be quantitatively detected as low as 29.3 nM. Cd²⁺ can enhance the fluorescence of probe 67 in 6.6 s, but its same group elements Cu²⁺ and Pb²⁺ can quench the fluorescence induced by Cd²⁺ to some extent. The researchers not only proposed the binding method of the probe 67 and Cd²⁺ by Job plot, FT-IR and ¹H NMR titration experiments, but also confirmed the accuracy of the probe in the detection of Cd²⁺ by the determination of Cd²⁺ in different water samples.

The probe 68 (Fig. 25), which was designed by conjugation of terpydine (a receptor for Zn²⁺ and Cd²⁺) and pyridinium with the styryl group, has good selectivity and anti-interference ability for Cd²⁺ recognition.⁷⁵ Probe 68 responds to Cd²⁺ based on the ICT and AIE mechanisms. In DMSO/H₂O system, upon excitation at 375 nm, the energy consumed by the intramolecular rotation of multiple pyridinium rings made the probe 68 emit weak fluorescence. As the water fraction in the solution increases to more than 80%, molecules aggregation and inhibition of intramolecular rotation made the probe 68 emit bright fluorescence. Adding Cd²⁺ made the fluorescence emission peak of the probe 68 red shift from 480 nm to 530 nm, but adding Zn²⁺ did not emerge obvious fluorescence spectral changes. W. X. Lin et al. introduced pyridinium group and alkane with sixteen carbon atoms into the probe 68 to increase its water solubility and cell membrane permeability, which can be used to detect and label Cd²⁺ in cells.⁷⁵

Fig. 25 The structure of probes 68 and 69.

The next year, P. P. Namitha's study team reported sensor 69 (Fig. 25).⁷⁶ In an ethanol solution and the excitation wavelength at 427 nm, the strong fluorescence of sensor 69 was quenched by Cd²⁺ due to the PET effect, and the fluorescence intensity ratio I₀/I has a linear relationship with Cd²⁺ of 0.25–2 μM. Ni²⁺, Zn²⁺ and Cu²⁺ can interfere with Cd²⁺ detection when the concentration exceeds 10 times of Cd²⁺ concentration, and when they exceed 100 times, they can slightly quench the fluorescence caused by Cd²⁺. The sensor 69 can detect Cd²⁺ as low as 0.1499 μM with excellent selectivity, light stability and water solubility, and can be used to measure Cd²⁺ in the NIR region.

In 2020, P. Liu et al. reported a fluorescent probe 70 (Fig. 26) based on ICT to detect Cd²⁺.⁷⁷ In HEPES buffer (1% DMSO as cosolvent) and the excitation wavelength at 380 nm, probe 70 has no colour and fluorescence response to cations other than Cd²⁺. The addition of Cd²⁺ turns the yellow weak fluorescence of probe 70 into red strong fluorescence, and the fluorescence intensity is linearly related to the Cd²⁺ concentration of 5.0 × 10⁻⁸ to 5.0 × 10⁻⁵ mol L⁻¹. The association constant between probe and Cd²⁺ was 7.97 × 10⁴ M⁻¹, and the detection limit for Cd²⁺ was 6.31 nM. In addition, the probe solution changes from yellow to maroon red by adding Cd²⁺, so it can be used as a new visual sensor for Cd²⁺. The probe 70 is easy to synthesize, and has excellent selectivity, sensitivity, reversibility (EDTA), stability, and a wide pH (3–13) range for Cd²⁺ detection. In addition, the probe 70 can be used for fluorescence imaging of Cd²⁺ in living cells.

Fig. 26 Proposed binding mode of probes 70 and 71 with Cd²⁺.

The sensor 71 (Fig. 26) designed by B. Jana's research team has high response efficiency to Cd²⁺ in water medium with pH of 1–10 and lower temperature.⁷⁸ In aqueous medium (water/DMSO, v/v: 95/5), sensor 71 has maximum absorption at 367 nm and 293 nm, the addition of Cd²⁺ makes maximum absorption shift from 293 nm to 313 nm. Under 367 nm, the addition of Cd²⁺ increases the fluorescence emission intensity of sensor 71 at 482 nm by 3 times, the fluorescence lifetime increases from 6.49 ns to 9.78 ns, and the fluorescence changes from colorless to faint blue. The dissociation constant (K_d) of Cd²⁺ and probe complex is 1.7 × 10⁻⁷, and the detection limit for Cd²⁺ is 0.95 μM. The fluorescence intensity (F₀/(F−F₀)) and Cd²⁺ (1/[Cd²⁺]) with a concentration of 0.05–1 μM has a linear relationship, which can be used to quantitatively detect Cd²⁺. However, the presence of Hg²⁺ can be reduced the fluorescence intensity of the complex by 9–10%.

In 2020, E. K. Inal et al. reported the sensor 72 (Fig. 27).⁷⁹ The ethanol–water solutions and the excitation wavelength at 369 nm are selected to carry out. The sensor 72 emits weak fluorescence at 464 nm, then addition of Zn²⁺/Cd²⁺/Hg²⁺ makes the sensor has obvious fluorescence emission at 452/474/491 nm, the fluorescence intensity and Zn²⁺/Cd²⁺/Hg²⁺ concentration has a linear relationship. The detection limit for Zn²⁺/Cd²⁺/Hg²⁺ is 2.7 × 10⁻⁷ M/6.0 × 10⁻⁷ M/7.5 × 10⁻⁷ M. The sensor 72 can detect Zn²⁺, Cd²⁺, Hg²⁺ in ethanol–water solutions with a pH of 8–9 (due to the protonation of nitrogen in imine bond, the fluorescence response to metal ions in acidic medium is very weak), but the sensor has a stronger affinity for Cu²⁺ and Fe³⁺. Moreover, Co²⁺ and Ni²⁺ interferes with the selectivity of the sensor 72 for Zn²⁺ and Cd²⁺.

Fig. 27 Proposed binding mode of probes 72 and 73 with Cd²⁺.

The research group of S. G. J. Andrews reported probe 73 (Fig. 27).⁸⁰ The coordination of probe 73 with Cd²⁺ or Pb²⁺ led to inhibit the PET process and produce the CHEF effect, so probe 73 emitted strong fluorescence. Probe 73 has selectivity, sensitivity, reversibility (EDTA) and visualization for the recognition of Cd²⁺ and Pb²⁺ in an aqueous buffer (<80%) medium, and is not affected by other metal ions or interferes with each other. In methanol/HEPES buffer (5 mM, v/v = 1 : 9, pH = 7.3), upon excitation at 420 nm, adding Cd²⁺ makes the emission peak of probe 73 redshift from 553 nm to 578 nm, and can detect Cd²⁺ as low as 12.96 nM in a pH system of 4.8–8.5. Upon excitation at 410 nm, adding Pb²⁺ makes the emission peak of probe 73 redshift from 499 nm to 505 nm, and can detect Pb²⁺ as low as 10.63 nM in a pH system of 5.0–8.4. The binding constants of probe for Cd²⁺ is 3.10 × 10⁵ M⁻¹ and for Pb²⁺ is 2.40 × 10⁵ M⁻¹. The probe 73 can be used as a naked-eye sensing probe for Cd²⁺ and Pb²⁺. Under visible light, the solution of the probe 73 changes from pale yellow to dark yellow and light pink, respectively. Under ultraviolet light, the probe solution changes from pale yellow to green and red, respectively. In addition, probe 73 can be used for fluorescence imaging of Cd²⁺ and Pb²⁺ in MCF 7 cells.

Y. F. Zhang et al. synthesized the sensor 74 (Fig. 28) by connecting 2-(2-((2-hydroxyphenyl)imino)ethylidene)amino unit (as the recognition site of Pb²⁺ and Cd²⁺) and tert-butyldiphenylsilyl (as the reaction center of F⁻, and improving the stability of the probe molecule and protecting the molecule from modification) to fluoresceins.⁸¹ Sensor 74 can recognize Pb²⁺ and Cd²⁺ quickly and sensitively by using F⁻ as auxiliary reagent. In EtOH/H₂O (99 : 1, v/v) sensor solution, adding Pb²⁺ or Cd²⁺ increased the absorption band at 533 nm and decreased the absorption band at 374 nm, resulting in the change of ratiometric spectral. When F⁻ is added to the sensor solution containing Pb²⁺ or Cd²⁺, the solution containing Pb²⁺ fades from light purple to colorless, and the solution containing Cd²⁺ deepens from light purple to dark purple. Through these phenomena, this sensor can be used as an indirect sensor to identify F⁻. The researchers believe that these phenomena are caused by the different radius of Pb²⁺ and Cd²⁺ and the different coordination mode with F⁻. The binding constants (K_a) of the sensor 74 with Pb²⁺ or Cd²⁺ are 3.292 × 10⁴ M⁻¹ and 2.349 × 10⁴ M⁻¹, respectively. The detection limits of sensor 74 for Pb²⁺ and Cd²⁺ are 0.42 μM and 0.53 μM, respectively.

Fig. 28 Proposed binding mode of probes 74 to 76 with Cd²⁺.

F. D. Carlos' team has developed an acridine based fluorophore sensor 75 (Fig. 28).⁸² In EtOH : CH₃Cl (99 : 1 v/v) solution, the combination of sensor 75 with Cd²⁺ suppressed the PET effect and enhanced the fluorescence. Under the excitation of 400 nm light, the fluorescence intensity of sensor 75 was enhanced by 746% with the addition of Cd²⁺, and was not affected by interference ions. The linear range of fluorescence intensity and Cd²⁺ concentration was 0.10–1.00 μmol L⁻¹. The binding constant of sensor 75 to Cd²⁺ is 1.05 × 10⁹ L mol⁻¹, and the detection limit and quantitative detection limit are 9.98 and 33.31 nmol L⁻¹, respectively. Sensor 75 has repeatability, reproducibility, robustness and reversibility (EDTA) for detecting Cd²⁺. In addition, sensor 75 can be used to detect Cd²⁺ in Brazilian sugarcane wine and tobacco smoke extract.

Rhodanine and salicylidene Schiff bases can bind to metal ions strongly, and have good photophysical properties and water solubility.⁸³ Therefore, the probe 76 (Fig. 28) was designed and synthesized by S. Park et al. When probe 76 was used to detect Zn²⁺ or Cd²⁺ in aqueous medium (λ_ex = 329 nm), the fluorescence intensity at 519 nm increased significantly.⁸³ However, the addition of cysteine can make the fluorescence induced by Cd²⁺ disappear, while the fluorescence induced by Zn²⁺ remain, so the distinction between Zn²⁺ and Cd²⁺ is realized. The binding constant of probe to Zn²⁺ or Cd²⁺ is 2.0 × 10⁷ M⁻¹ or 1.0 × 10⁷ M⁻¹, and the detection limit for Zn²⁺ or Cd²⁺ is 1.07 μM or 1.37 μM. The probe 76 can respond to Zn²⁺ (detection is affected by Co²⁺, Cu²⁺, Fe³⁺, Fe²⁺, Na⁺) or Cd²⁺ (detection is affected by Co²⁺, Cu²⁺, Fe³⁺, Fe²⁺, Na⁺, Zn²⁺) in the pH range of 7.0–9.0 for water sample analysis and cell imaging, but its anti-interference ability is poor.

Because thiosemicarbazide has many coordination modes with metal ions, P. S. Kumar et al. synthesized the probe 77 (Fig. 29) by connecting it to the fluorophore unit (4-tert-butyl-2,6-diformylphenol).⁸⁴ The probe can be used for the fluorescence detection of Zn²⁺, Cd²⁺ and Hg²⁺ in H₂O : DMSO (95 : 5 v/v) medium and 360 nm excitation wavelength. The fluorescence intensity ratios I_{488 nm}/I_{540 nm} and I_{470 nm}/I_{540 nm} were linearly correlated with the concentrations of Zn²⁺ and Cd²⁺, respectively. Hg²⁺ makes the probe emit weak reddish brown fluorescence near 578 nm. The binding constants of Zn²⁺, Cd²⁺ and Hg²⁺ complexes are 9.8 × 10³, 1.39 × 10⁵ and 2.03 × 10¹³ M⁻¹, respectively. The binding ability of the probe to Zn²⁺, Cd²⁺ and Hg²⁺ increased in turn, and the metal ions with strong binding ability could interfere with the detection of metal ions with weak binding ability. Probe 77 can respond to Zn²⁺, Cd²⁺ and Hg²⁺ in the pH range of 3–10, with detection limits as low as 5.1, 3.4 and 0.51 μM.

Fig. 29 Proposed binding mode of probes 77 to 79 with Cd²⁺.

Z. Aydin et al. reported the Schiff base probes 78a and 78b (Fig. 29) with N-phenyl-o-phenylenediamine as Cd²⁺ binding part and benzaldehyde and cinnamaldehyde as chromophores.⁸⁵ For the recognition of Cd²⁺ ions in aqueous solution, the probe 78a changes from colorless to orange, and the probe 78b changes from yellow to reddish. These probes can quantitatively detect Cd²⁺ based on absorbance. Although the ions has no effect on the colorimetric detection of Cd²⁺ by 78b, its spectrometric titration is interfered by Zn²⁺. The binding constants of 78a and 78b with Cd²⁺ were 2.65 × 10¹² M⁻² and 4.95 × 10¹² M⁻², respectively. The detection limits of 78a and 78b were 4.38 × 10⁻⁷ M and 1.02 × 10⁻⁷ M, respectively. The two probes have selectivity and reversibility for the recognition of Cd²⁺, and can be used as colorimetric sensors for the detection of Cd²⁺ in aqueous solution.

J. J. Celestina et al. rapidly synthesized the colorimetric and fluorescent sensor 79 (Fig. 29) for Cd²⁺ from 4- aminoantipyrine and 9-anthracene carboxaldehyde under microwave irradiation.⁸⁶ The addition of Cd²⁺ makes the sensor produce PET-CHEF effects and inhibit the C=N isomerization, resulting in the solution changes from yellow to light brown and the fluorescence changes from yellow to green. The detection limit of the sensor 79 for Cd²⁺ is as low as 0.02 μM, and the detection is reversible and specific. In addition, sensor 79 can be used for the detection of Cd²⁺ in water samples and fluorescence imaging in L929 cells.

P. Lasitha et al. synthesized a squaramine ring based probe 80 (Fig. 30), which can respond to Zn²⁺ and Cd²⁺ by turning on fluorescence.⁸⁷ Under the excitation of 400 nm light, the maximum emission peak of the probe shifts from 464 nm to long wavelength with the addition of Zn(CH₃COO)₂ and Cd(CH₃COO)₂. With the addition of Zn(CH₃COO)₂, a new emission band appeared at 560 nm, and gradually red shifted to 595 nm. Similarly, with the addition of Cd(CH₃COO)₂, a new emission band appeared near 530 nm, and gradually red shifted to 540 nm. The order of stokes shift emission is the same as that of anion basicity (ClO₄⁻ < Cl⁻ < NO₃⁻ < SO₄²⁻ < CH₃COO⁻). The detection limits for Zn(CH₃COO)₂ and Cd(CH₃COO)₂ were 5 × 10⁻⁷ M (32.6 ppb) and 7 × 10⁻⁸ M (7.8 ppb), respectively.

Fig. 30 The structure of probes 80 to 82.

M. Muddassir's team prepared a complex sensor 81 (Fig. 30) of 3,5-dimethylpyrazole with Cu²⁺ perchlorate.⁸⁸ Sensor 81 can identify Cd²⁺ in aqueous solution by fluorescence enhancement, and is not interfered by other metal ions. M. Muddassir et al. confirmed the binding of the complex with Cd²⁺ by X-ray photoelectron spectroscopy (XPS) analysis.⁸⁸ They speculate that the high selectivity and sensitivity to Cd²⁺ may be due to the interaction between Cd²⁺ and 3,5-dimetylpyrazole ligand through Lewis acid–base interactions, which promotes more effective energy transfer from ligand to Cu²⁺. In addition, the complex probe can also respond to acetone by fluorescence quenching.

Vanillinyl-hydrazone Schiff base probe 82 (Fig. 30) is a turn-on fluorescent probe for Mg²⁺, Zn²⁺, Cd²⁺ and I⁻, which was reported by S. Dey's team in 2020.⁸⁹ In DMSO/H₂O medium (9 : 1) and under 480 nm excitation wavelength, the different emission wavelengths and fluorescence colors of this probe can be used as the methods to distinguish Zn²⁺ (λ_em = 546 nm, yellow green), Cd²⁺ (λ_em = 576 nm, yellow) and Mg²⁺ (λ_em = 625 nm, orange). The binding constants of probe 82 to Zn²⁺, Cd²⁺ and Mg²⁺ are 8.07 × 10⁴ M⁻¹, 6.5 × 10⁴ M⁻¹ and 4.38 × 10⁴ M⁻¹, respectively. The detection limits for Zn²⁺, Cd²⁺ and Mg²⁺ were 14 × 10⁻⁹ M, 6.8 × 10⁻⁹ M and 11.5 × 10⁻⁹ M, respectively. In different detection environments, probe 82 can also be used as a fluorescent probe for I⁻, and the detection limit is as low as 5.9 nM.

Using fluorene as fluorophore, 2,2′,6′,2′′-terpyridine as ligand and benzothiazole as electron acceptor, D. Yue et al. synthesized probe 83 (Fig. 31) with high electron absorption, fluorescence quantum yield and photostability.⁹⁰ They also introduced 2-(2-ethoxyethoxy)-ethyl group to enhance the solubility of the probe. Zn²⁺ and Cd²⁺ were identified by different emission colors in acetonitrile. The detection limits for Zn²⁺ and Cd²⁺ were 11.25 nM and 1.80 nM, respectively.

Fig. 31 The structure of probe 83.

The spectroscopic and analytical parameters of other Cd²⁺ sensors based on small organic molecules are listed in Table 7. There are many problems in these probes, such as low sensitivity, poor water solubility and weak anti-interference ability. Some probes recognize Cd²⁺ only through colorimetric response, and they generally have high selectivity and anti-interference, but their sensitivity is not high (the minimum detection limit is 102 nM). Fluorescent sensors usually have high sensitivity, but in these reports, there are relatively a few sensors with detection limits as low as 10⁻⁷ M. Few sensors can meet the requirements of water solubility, selectivity, sensitivity, anti-interference and practicability (such as appropriate pH, temperature and response time).

Table 7 Spectroscopic and analytical parameters of other small organic molecules based Cd²⁺ fluorescent and colorimetric probes

Probe	Solvent	Excitation wavelength (nm)	Emission wavelength (nm) λem0–λem	Detection limit (nM)	Association constant	Interfering ion(s)	Ref.
44	CH3CN–H2O (1 : 5)	400	558–561	10.21	3.16 × 10^5 M^−1	—	12
45	CH3CN	410; 495	675–570; 730–570	6.9	—	Zn^2+	19
46	H2O	418	653–611	32	Kd = 31.2 ± 5.2 μM	Hg^2+, Cu^2+	60
47	EtOH–H2O (9 : 1)	470	545–537	57.6	—	Many	17
48	NPA/CYH (1 : 30)	495	518–538	2	—	Co^2+, Cu^2+, Zn^2+, Hg^2+	61
49	DMF	—	—	200	3.7 × 10^4 M^−1	—	62
50	EtOH – EAC (7 : 3)	283	510	2690	—	—	63
51	CH3CN–H2O (1 : 99)	315	433	100	3862.872 M^−1	—	64
52	MeOH	308	450	50	Kb = 4.87	IO4^−, Zn^2+	66
53a/b	DMSO; DMF	—	—	3220; 200	—	—; Hg^2+, Cu^2+, Pb^2+	67
54	MeOH	320	418–393	1020	—	Al^3+	68
55	CH3CN	378	452	940	(7.49 ± 0.18) × 10^5 M^−1	Zn^2+	69
56	DMSO–H2O (9 : 1)	482	560	6.6	0.96 × 10^4 M^−1	Zn^2+, I^−	13
57	MeOH–H2O (7 : 3)	435	495–540	1640	—	Cu^2+	10
58	DMF–H2O (9 : 1)	323	412–486	861	4.98 × 10^4 M^−1	—	5
59	EtOH–H2O (1 : 1)	340	473	26.2	—	Fe^2+	70
60a/b	EtOH–H2O (1 : 1)	277	368/474	—	—	—	71
61	MeOH	395	572	174	—	Cu^2+, Co^2+, Zn^2+	26
62	H2O	528	600	520	(6.0 ± 0.3) × 10^4 M^−1	Zn^2+, Co^2+, Cu^2+	14
63	CH3CN–H2O (99 : 1)	340	530	600	1.36 × 10^6 M^−2	CN^−	72
64	MeOH	—	—	14 688	4.29 × 10^6 M^−1		73
65	H2O	280	355/410	—	—	Zn^2+	29
66	EtOH–H2O (4 : 1)	—	—	9000–10 000	—	Co^2+	30
67	DMF–H2O (3 : 7)	275	377	29.3	3.15 × 10^5 M^−1	Cu^2+, Pb^2+	74
68	DMSO–H2O	375	480–530	—	—	Fe^3+, Co^2+, Cu^2+, Pb^2+	75
69	EtOH	427	656	149.9	—	Ni^2+, Zn^2+, Cu^2+	76
70	DMSO–H2O (9 : 1)	380	513–515	6.31	7.97 × 10^4 M^−1	—	77
71	DMSO–H2O (5 : 99)	367	482	950	Kd = 1.7 × 10^−7	Hg^2+	78
72	EtOH–H2O	369	464–474	600	—	Cu^2+, Fe^3+, Co^2+, Ni^2+	79
73	MeOH–H2O (1 : 9)	420	553–578	12.96	3.10 × 10^5 M^−1	Pb^2+	80
74	EtOH–H2O (99 : 1)	—	—	530	2.349 × 10^4 M^−1	Pb^2+	81
75	EtOH–CH3Cl (99 : 1)	400	416	9.98	1.05 × 10^9 L mol^−1	—	82
76	H2O	329	519	1370	1.0 × 10^7 M^−1	Co^2+, Cu^2+, Fe^3+, Fe^2+, Na^+, Zn^2+	83
77	DMSO–H2O (5 : 99)	360	578	3400	1.39 × 10^5 M^−1	Zn^2+, Hg^2+	84
78a/b	H2O	—	—	438/102	2.65 × 10^12 M^−2/4.95 × 10^12 M^−2	—	85
79	EtOH	398	345–404 and 465	20	—	Many	86
80	DMSO	400	530–540	70	—	Zn^2+	87
81	CH3CN–H2O	267	650	—	—	—	88
82	DMSO–H2O (9 : 1)	480	576	6.8	6.5 × 10^4 M^−1	Zn^2+, Mg^2+, I^−	89
83	CH3CN	410	460	1.8	—	Zn^2+	90

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9. Nanosensor-based Cd²⁺ fluorescent sensor

Compared with small organic molecules fluorescent probes, the nanosensor-based fluorescent and colorimetric probes on avoid complex synthesis and purification processes.⁹¹ In addition, nanosensors have the characteristics of wide excitation wavelength, adjustable size, high quantum yield and good photochemical stability.^91,92 Therefore, the development of nanosensors in Cd²⁺ fluorescent probes is also worthy of our attention.

In 2017, Q. Wu's team first reported a probe 84 that enhances the fluorescence of the second near-infrared (NIR-II) region based on quantum dots identification Zn²⁺ and Cd²⁺.⁹³ The research group prepared oil-soluble Ag₂S QDs by capping Ag₂S nanocrystals with n-dodecyl mercaptan groups. However, due to the limited application of the oil-soluble probe, they replaced dodecyl mercaptan as a surface ligand with thioglycollic acid containing carboxyl group to obtain water-soluble probe 84. The UV-vis absorption spectrum of probe 84 has no obvious peak in the range of 250–910 nm, and the fluorescence emission intensity gradually decreases with the increase of excitation wavelength. After adding Zn²⁺ or Cd²⁺, thioglycollic acid on the surface of probe 84 reacts with ions to form Zn–thiol or Cd–thiol complex passivation shell around the surface of Ag₂S quantum dots. Due to the passivation shell repairing surface defects and inhibiting non-radiative recombination pathway, the fluorescence intensity of probe 84 at 1100 nm increased by 7 or 8.5 times under 480 nm light excitation, and the fluorescence lifetime increased from 3.25 ns to 7.35 or 10.42 ns, respectively. The fluorescence intensity ratio (I/I₀) of probe 84 has a linear relationship with the concentration (1–40 μM) of Zn²⁺ or Cd²⁺, and can be used for quantitative detection of Zn²⁺ or Cd²⁺. The detection limits of probe 84 for Zn²⁺ and Cd²⁺ are 760 nM and 546 nM, respectively. In addition, the probe 84 has strong practicability. It can not only detect Zn²⁺ or Cd²⁺ in lake water and tap water samples, but also be used for cell imaging of Zn²⁺ in NIR-II region.

According to the interaction between C=N group and specific metal ions, J. T. Wang et al. obtained 3D porous MOF-UiO-66-N=CH₂ with perfect octahedral morphology and particle size of about 100 nm through the interaction of –NH₂ of UiO-66-NH₂ with formaldehyde.⁹⁴ MOF-UiO-66-N=CH₂ was used as the fluorescent probe 85 of Cd²⁺. In aqueous solution and light excitation at 342 nm, probe 85 has an emission peak at 468 nm. When metal ions were added to probe 85, only Fe³⁺ quenched its fluorescence intensity, Cd²⁺ and Zn²⁺ increased its fluorescence intensity, and the effects of other metal ions could be neglected. However, the increase of luminescence intensity of probe 85 caused by Cd²⁺ is the strongest, and the fluorescence intensity ratio (I/I₀) has a good linear relationship with Cd²⁺ concentration (0–500 μM). Therefore, probe 85 can be used as a fluorescence on probe for quantitative detection of Cd²⁺, the detection limit is as low as 0.336 μM (37.8 ppb), and the K_SV value is 3.81 × 10³ M⁻¹. In addition, probe 85 has low-toxicity and pH (4–10) independence, and is promising for Cd²⁺ detection in water and intracellular environments.

In 2017, L. Li's research team reported a fluorescent probe 86 based on dithizone (DZ) etched cadmium telluride nanoparticles (CdTe NPs) for the detection of Cd²⁺.⁹² After DZ was added to L-cysteine-capped CdTe NPs, the Cd–thiol complex surface layers was destroyed and the surface passivation was reduced due to the chemical etching effect of DZ, resulting in the reduction of fluorescence intensity and fluorescence lifetime. After adding Cd²⁺ to the weakly fluorescent CdTe NPs–DZ mixture (probe 86), the recognition site of Cd²⁺ on the surface of NPs can selectively bind Cd²⁺ to re-passivate the surface of NPs, resulting in the increase of fluorescence intensity and fluorescence lifetime. Under ultrapure water medium and excitation wavelength of 328 ± 5 nm, DZ or Cd²⁺ can be quantitatively detected within 40 min according to the fluorescence intensity (468 nm) ratio before and after adding DZ or Cd²⁺. The detection limit for DZ is 2.7 μM, and the detection limit for Cd²⁺ is 0.13 μM. In addition, Ag⁺ and Cu²⁺ almost completely quench the fluorescence of non-etched and DZ etched NPs. However, thiourea can be selected as a masking agent to eliminate the interference of Ag⁺ and Cu²⁺.

In 2017, Y. Yang et al. reported a super stable Eu-MOF luminescent probe 87. The probe 87 has extremely high thermal stability, air stability and chemical stability, and has fluorescence response to multiple analytes.⁹⁵ The luminescence sensing of metal ions by probe 87 was studied in aqueous solution and 324 nm excitation wavelength. It was found that the fluorescence intensity of probe 87 at 613 nm gradually increased (2.18 times)/decreased (about 85%) with the addition of Cd²⁺/Mn²⁺. According to Stern–Volmer (SV) equation: I₀/I = 1 + K_SV[M], the quenching effect coefficients (K_SV) of probe 87 for Cd²⁺ and Mn²⁺ are −54 or 443 L mol⁻¹ respectively, indicating that Mn²⁺ has high fluorescence quenching efficiency for probe 87. Probe 87 can detect Cd²⁺ and Mn²⁺ as low as 10⁻⁶ M. The research team speculated that the increase or quenching of fluorescence of probe 87 caused by Cd²⁺/Mn²⁺ may be due to the cation exchange between free [Me₂NH₂]⁺ and target ions and the interaction between metal ions and organic ligands. In addition, methanol or ether can significantly enhance or weaken the emission of probe 87 at 613 nm, which is the first time to use Ln-MOFs as a luminescent probe to identify ether molecules.

Because biomass contains a lot of carbohydrates, it can be used as raw material for preparing carbon dots (CDs). In 2018, D. Gu's team synthesized a nitrogen and sulfur co-doped sensor 88 with scallion as carbon source, which can detect Cd²⁺ by fluorescence quenching.⁹⁶ The response of sensor 88 to Cd²⁺ was studied in HAC–NaAc buffer solution (pH = 5.0) and 390 nm excitation wavelength. The linear relationship can be established between the emission intensity ratio of sensor 88 at 455 nm and Cd²⁺ concentration of 0.1–3.0 μM or 5.0–30.0 μM respectively, and the corresponding binding constants were 2.816 × 10⁵ ± 11 670 L mol⁻¹ or 3.636 × 10⁴ ± 2120 L mol⁻¹ respectively. The sensor 88 has no obvious fluorescence response to other metal ions except Cd²⁺, and the fluorescence quenched by Cd²⁺ is hardly disturbed by other metal ions. It can detect Cd²⁺ as low as 15.0 nM. In addition, the sensor 88 has extremely low cytotoxicity and detection limit, and has a good application prospect for Cd²⁺ ion sensing and biological imaging in vivo or in vitro.

Y. A. Pan et al. selected a semi-rigid ligand with carboxyl group (2,5-bis(phenylamino)-1,4-benzenedicar boxylic acid, L) as an aromatic organic linker and selected Zn²⁺ ions with outer shell electronic configuration similar to Cd²⁺ as ion exchange interaction sites to construct a new three-dimensional Zn-MOF sensor 89.⁹⁷ Since Cd²⁺ replaces Zn²⁺ and reacts with the carboxyl group of L, a more stable Zn–Cd-MOF structure is obtained, which limits the rotation of L in sensor 89 and improves the rigidity of the whole system, the fluorescence of probe 89 increases when identifying Cd²⁺. Under 365 nm UV irradiation, it can be seen that the probe 89 turns on yellow fluorescence induced by Cd²⁺. When excited at 333 ± 5 nm, probe 89 has a fluorescence emission peak at 529 nm. After adding Al³⁺ and Cd²⁺ ions, the fluorescence emission of probe 89 is enhanced, but Al³⁺ causes the emission peak of probe 89 to red-shift to 555 nm without interfering with the quantitative detection of Cd²⁺ at 529 nm according to the fluorescence intensity ratio I/I₀. The fluorescence of probe 89 is stable within 20 min after adding Cd²⁺, and the presence of Cu²⁺, Mn²⁺, Ni²⁺ and Co²⁺ in the detection system will have a slight impact on the determination of Cd²⁺. In addition, the probe 89 can recognize nitrobenzene by fluorescence quenching, and the detection limit is 1.19 μg mL⁻¹.

Z. Y. Zhang's research team reported a graphite-like nitride doped carbon quantum dots-capped gold nanoparticle (Au@g-CNQDs, probe 90).⁹⁸ Because the surface of probe 90 has numerous heptazine, carboxyl and hydroxyl groups, a trace amount of Cd²⁺ ions can be adsorbed on the probe 90 surface through “cooperative effect”. In Tris–HCl buffer solution, the color of probe 90 changes from wine-red to blue-violet within 30 s after the addition of Cd²⁺, and the absorption peak of probe 90 decreases at 520 nm and appears at 650 nm. The absorption ratio (A_{650 nm}/A_{520 nm}) of probe 90 has a linear relationship with Cd²⁺ concentration (0.01–3.0 μM), and Cd²⁺ can be detected quantitatively with a detection limit of 10 nM. In addition, the probe 90 can accurately and quickly detect the distribution of Cd²⁺ ions in mouse organs and tissues, indicating that it has considerable application potential in practical application and clinical detection.

Through the reaction of chelidamic acid-picolylamine (CP) substituted diacetylene monomer and the reaction between 10,12-pentacosadiynoic acid and CP chelating part, P. Thanh Chung et al. synthesized probe 91 based on conjugated polyacetylene.⁹⁹ In HEPES buffer (10 mM, pH = 7.4) and excitation wavelength of 530 ± 5 nm, the addition of Cd²⁺ rapidly turned on the fluorescence of probe 91, and the fluorescence emission intensity increased with the concentration of Cd²⁺ ion from 0 to 300 μM. In addition, since two pyridine and chelidamic acid moieties of probe 91 chelating Cd²⁺ shortens the effective conjugate chain length of probe 91, different amounts of Cd²⁺ caused the colorimetric change of probe 91 from blue to red(violet). Probe 91 binds to Cd²⁺ in a stoichiometric ratio of 2 : 1, and the LOD for Cd²⁺ is as low as 1.85 × 10⁻⁵ M.

M. Y. Tang et al. reported probe 92 of nitrogen sulfur doped carbon dots (N&S CDs) with high water solubility, selectivity, sensitivity and stability to Cd²⁺.¹⁰⁰ Using triethylenetetramine (TETA) and 2-mercaptothiazoline (2-MT) as raw materials can significantly improve the quantum yield of CDs, and they have rich sulfhydryl and amino groups that can provide more opportunities for application implementation. When excited at 340 nm, the probe 92 can quickly (2 min) recognize Cd²⁺ in ultrapure water and real water samples and quench the bright blue fluorescence at 459 nm. Because the fluorescence quenching of the probe caused by the potential aggregation of Cd²⁺ and sulfhydryl and amide groups on the surface of N&S CDs. Probe 92 has no obvious reaction with metal ions, biomolecules and anions other than Cd²⁺, and the presence of other metal ions in the detection system also has no obvious interference with the detection of Cd²⁺. The probe 92 can quantitatively detect the Cd²⁺ concentration of 0.02–1 μM, and the detection limit is 0.018 μM. In addition, the probe 92 not only has low toxicity and good cell permeability, but also remains stable in the pH range of 4–12 (the best detection pH is 9).

C. Wei's team reported the first quaternary QDs (Zn–Ag–In–S QDs) probe 93 based on aggregation-induced emission enhancement (AIEE) mechanism.⁹¹ Under the phosphate buffer solution (pH = 7.4) and the excitation wavelength of 360 nm, the probe 93 reacts with metal ions for 20 min before fluorescence detection. Since L-cysteine is used as a stabilizer and ligand of Zn–Ag–In–S QDs, it can coordinate with Cd²⁺ through thiol groups, resulting in enhanced fluorescence emission and blue-shifted the emission peak of probe 93, and formed macroscopic floc in the presence of high concentration of Cd²⁺. Because the coordination of thiol groups with Cd²⁺ weakens the electrostatic repulsion between QDs, forms microheterojunctions and passivates surface defects, so the probe 93 shows enhanced fluorescence. In addition, Cd²⁺ may diffuse into Zn–Ag–In–S QDs to widen the band gap, and the emission peak of probe 93 is blue shifted at 520 nm. Probe 93 can quantitatively detect Cd²⁺ (25 μM to 2 mM), and the detection limit is 1.56 μM. In addition, the presence of Pb²⁺, Hg²⁺ and Cu²⁺ can also cause the fluorescence change (quenching the fluorescence) of probe 93. The effects of Pb²⁺ and Hg²⁺ are eliminated by adding ammonium fluoride and thiosemicarbazide as masking agents, and the effects of Cu²⁺ are decreased by chelating Cu²⁺ with L-cysteine in advance.

In 2020, according to the hard-soft acid base (HSAB) principle, the soft donor site can react with soft toxic metals. M. Arnab's team constructed a three-dimensional columnar layered Zn-MOF sensor 94 using bis(pyridin-4-ylmethylene)benzene-1,4-diamine (as the Cd²⁺ ligand), thiophene-2,5-dicarboxylic acid and Zn(NO₃)₂.¹⁰¹ When Cd²⁺ was added to the CH₃CN solution of sensor 94, it can be observed by the naked eye that the color immediately changes from a very faint green colour to a deep orange. Upon the excitation at 330 nm, the addition of Cd²⁺ combined the emission peaks of sensor 94 at 390 nm and 468 nm into one emission peak at 446 nm, and the blue emission peak moved to 408 nm with the increase of the amount of Cd²⁺. In addition, the addition of Cd²⁺ enhanced the fluorescence lifetime and fluorescence intensity (408 nm) of sensor 94. The detection limit of sensor 94 for Cd²⁺ is 0.132 μM. Moreover, by repeatedly cleaning the sensor 94 that has been used for Cd²⁺ detection, the reversibility of the sensor can be realized.

Z. Q. Zhou's team reported a fluorescent probe 95, which turns on fluorescence to identify Cd²⁺ by replacing Mn²⁺ near the surface of Mn:ZnSe QDs with Cd²⁺.¹⁰² The doping of Mn²⁺ near the QD surface reduces the fluorescence intensity of probe 95. Due to the similar ion radius between Cd²⁺ and Zn²⁺, the external Mn²⁺ is replaced by Cd²⁺, resulting in the probe 95 emitting strong orange fluorescence and the fluorescence intensity increases by 20 times. The researchers optimized the detection conditions of probe 95 for the detection of Cd²⁺ in aqueous medium, such as the optimal concentration of probe 95 is 0.01 mg mL⁻¹, the optimal pH is 7 (higher pH is conducive to the deprotonation and electronegativity of MPA, so as to enhance the electrostatic attraction with Cd²⁺) and the optimal reaction time is 15 min. Strong orange fluorescence of probe 95 was observed only when incubated with Cd²⁺. Under the excitation of 367 nm light, the fluorescence intensity at 600 nm is linear with Cd²⁺ concentration (0.02–60 μM), and Cd²⁺ as low as 18 nM can be detected.

P. Subhash Chandra et al. synthesized carbon dots sensor 96 using curry leaves as carbon source for the first time.¹⁰³ Since the amine group in probe 96 provides donate an electron pair to excited state vacant orbital of the Cd²⁺ to form ligand-to-metal charge transfer (LMCT) process, the sensor recognizes Cd²⁺ by fluorescence off. The addition of Cd²⁺ not only reduced the fluorescence intensity and fluorescence lifetime of probe 96, but also shifted its absorption peak and emission peak to lower wavelengths by about 10 nm. At the excitation wavelength of 390 nm, Cd²⁺ was added to the aqueous solution of probe 96 for 15 min, and the fluorescence emission of probe 96 at 450 nm was detected. The fluorescence intensity ratio at 450 nm before and after adding Cd²⁺ to probe 96 has a good linear relationship with the concentration of Cd²⁺ (0.01–8 M), and Cd²⁺ as low as 0.29 nM can be detected quantitatively. Other metal ions do not affect the fluorescence emission of probe 96 or the fluorescence response of probe 96 to Cd²⁺. However, due to the presence of phenol (–OH) and carbonyl groups (C=O, positive charge carrier) on the surface of the probe 96, the fluorescence properties of the probe are pH dependent.

T. Alizadeh's team synthesized a fluorescent imprinted polymer probe 97 by a combination of hydrophilic functional monomers and hydrophobic ligands.¹⁰⁴ The luminescent complexing agent 5-((3hydroxynaphthalen-2-yl)methylene)pyrimidine-2,4,6(1,3,5)-trione (HMPT) forms a stable complex with Cd²⁺, then the mixture of acryl amide, vinyl benzene and ethylene glycol dimethacrylate are added for copolymerization, and finally Cd²⁺ is extracted from the polymer, resulting in a cavity, which can be selectively recombined with Cd²⁺. At the excitation wavelength of 365 nm, Cd²⁺ was added to turn on the high-intensity red fluorescence of probe 97, and the emission peak blue-shifted from 440 nm to 502 nm. After the probe 97 reacted with Cd²⁺ for 20 min, the emission intensity at 502 nm had a good correlation with the concentration of Cd²⁺ (10 to 0.05 mM). The limit of detection (LOD) and limit of quantification (LOQ) of probe 97 for Cd²⁺ were 12.3 and 41 nM, respectively. Probe 97 not only has high selectivity, but also has low cytotoxicity. It also can be used as a fluorescent probe for the detection of Cd²⁺ in living cells.

In 2021, W. T. Li et al. reported a dual-emission ratiometric fluorescence probe 98, in which silicon oxide-coated copper nanoclusters (CuNCs@SiO₂) as signal reference and cadmium telluride quantum dots (CdTe QDs) as signal response.¹⁰⁵ Since SiO₂ coating prevents the reaction of CuNCs with other substances, it is ensured that CuNCs@SiO₂ can provide a stable and reliable reference signal. In addition, based on the strong affinity of the two nitrogen atoms of 1,10-phenanthroline (Phen) for Cd in CdTe QDs, the researchers prepared CdTe QDs–Phen complex probe. Due to the photo-induced hole transfer (PHT) between CdTe QDs and Phen, the fluorescence of CdTe QDs–Phen complex probe is weak. After adding Cd²⁺, Phen separated from CdTe QDs–Phen complex and formed [Cd(Phen)₂(H₂O)₂]²⁺ and CdTe QDs, so the fluorescence intensity of CdTe QDs recovered. Under the excitation of 365 nm light, the characteristic fluorescence emission peaks of CuNCs@SiO₂ and CdTe QDs are well separated at 440 nm and 580 nm respectively, and the addition of Cd²⁺ only increases the fluorescence of CdTe QDs at 580 nm. Therefore, the Cd²⁺ content can be quantitatively analyzed according to the fluorescence intensity ratio (I₅₈₀/I₄₃₆) of probe 98 between 580 nm and 436 nm. The ratio fluorescent probe 98 can be sensitive with a detection limit of 1.1 μg L⁻¹ (2.75 μg kg⁻¹) in a detection time of 6 min, and the detection can eliminate background interference, with high sensitivity and accuracy. In addition, the fluorescent color captured by the smartphone camera can be converted into digital values representing red, green and blue (RGB) color modes through the Color Picker app, and Cd²⁺ can be quantitatively analyzed through the ratio (G/R) of green (G) and red channels (R).

Table 8 shows the spectroscopic and analytical parameters of Cd²⁺ recognition based on nanosensors. Sensors based on MOF, QDs, nanoclusters and nanoparticles have achieved good development in recent years. For example, probe with enhanced near-infrared fluorescence emission are reported for the first time, AIE sensor based on quaternary quantum dots are reported for the first time, and quantum dot probe are synthesized using biomass as carbon source, etc. These nanosensors have good water solubility, but their selectivity, sensitivity, anti-interference ability and practicability (such as pH and response time) still need to be improved. These nanosensors can identify Cd²⁺ ions by chemical etching reaction, displacement reaction, cavity-imprinting, chelation passivation on the surface of nanoparticles and other methods. Researchers can design and improve the performances of nanosensors based on these mechanisms. In addition, the MOF sensor 85 based on C=N recognition of Cd²⁺, the quaternary quantum dot sensor 93 synthesized with cysteine as the ligand of Cd²⁺, and the ratiometric sensor 98 with 1,10-phenanthroline chelating Cd²⁺ to restore CdTe fluorescence. These reaction sites are similar to the design of small organic molecules sensors. Therefore, the idea of constructing and improving nanosensors can also be obtained based on the research results of small organic molecules probes in Cd²⁺ recognition.

Table 8 Spectroscopic and analytical parameters of the nanosensor based Cd²⁺ fluorescent and colorimetric probes

Probe	Luminous structure	Solvent	Excitation wavelength (nm)	Emission wavelength (nm) λem0–λem	Detection limit (nM)	Interfering ion(s)	Ref.
84	Ag2S QDs	H2O	480 (250–910)	1100	546	Zn^2+	93
85	Zr-MOF	H2O	342	486	336	Fe^3+, Zn^2+	94
86	CdTe NPs	H2O	328	468	130	Ag^+, Cu^2+	92
87	Eu-MOF	H2O	324	613	1000	Mn^2+	95
88	CDs	H2O	390	455	15	—	96
89	Zn-MOF	H2O	333	529	1.19 μg mL^−1	Cu^2+, Mn^2+, Ni^2+, Co^2+	97
90	Au@g-CNQDs	H2O	—	—	10	—	98
91	Polydiacetylene	H2O	530	About 625	18 500	—	99
92	N&S CDs	H2O	340	459	18	—	100
93	Zn–Ag–In–S QDs	H2O	360	520	1560	Pb^2+, Hg^2+, Cu^2+	91
94	Zn-MOF	CH3CN	330	390 and 468–446 -408	132	—	101
95	Mn:ZnSe QDs	H2O	367	600	18	—	102
96	CDs	H2O	390	450	0.29		103
97	Imprinted polymer	CH3CN	365	440–502	12.3 (LOQ = 41)	—	104
98	CuNCs@SiO2; CdTe QDs	H2O	365	580 and 436	1.1 μg L^−1	—	105

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10. Conclusions and outlooks

In this paper, we summarized the reports of Cd²⁺ fluorescent and colorimetric probes with different fluorophores since 2017, and found that there are still some problems to be solved in the practical application of Cd²⁺ fluorescent sensors. For example, the specific selectivity and anti-interference ability of most probes remain poor, which greatly affect the qualitative and quantitative detection of Cd²⁺ in complex environment. Secondly, the water solubility of many probes are poor, which inevitably result in addition of organic solvent into the detection medium and then pollution formation. Thirdly, the detection sensitivity of some probes are not enough to meet the maximum allowable concentration of Cd²⁺ in drinking water stipulated by WHO. Fourthly, few of the probes can realize near-infrared recognition of Cd²⁺, and most of the probes only identify Cd²⁺ in the blue and green light regions, which are easily interfered by the background fluorescence of biological samples and limits their application in biological system.

Therefore, the design and development of Cd²⁺ fluorescent probes in the future, on the one hand, need to discover and optimize the novel recognition groups with high selectivity, sensitivity and water solubility. On the other hand, overall consideration of the economy of raw materials, the simplicity of preparation, the practicability and feasibility of the probes, should be paid more attention. Finally and most importantly, the recognition pattern and mechanism of the probes should be elucidated clearly. Only in this way can we effectively perform the molecular structure design and find the lead compound with high performance.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Science and Technology Major Project of China (No. 2017ZX05049-003-007) and the National Natural Science Foundation of China (No. 21801022).

Notes and references

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