A novel and fast responsive turn-on fluorescent probe for the highly selective detection of Cd²⁺ based on photo-induced electron transfer

Abstract

A novel, highly sensitive and fast responsive turn-on fluorescence probe, 2,2′-((1E,1′E)-((1,10-phenanthroline-2,9-diyl)bis(methanylylidene)) bis(azanylylidene)) diphenol (ADMPA), for Cd²⁺ was successfully developed based on 2,9-dimethyl-1,10-phenanthroline and o-aminophenol. ADMPA showed a remarkable fluorescence enhancement toward Cd²⁺ against other competing cations, owing to the suppression of the photo-induced electron transfer (PET) and CH=N isomerization. A good linear relationship (R² = 0.9960) was obtained between the emission intensity of ADMPA and the concentration of Cd²⁺ (0.25–2.5 μM) with a detection limit of 29.3 nM, which was much lower than that reported in literature. The binding stoichiometry between ADMPA and Cd²⁺ was 2 : 1 as confirmed by the Job's Plot method, which was further confirmed by a ¹H NMR titration experiment. Moreover, the ADMPA probe was successfully applied to detect Cd²⁺ in real water samples with a quick response time of only 6.6 s, which was about 3–40 times faster than the reported cadmium ion probe.

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Introduction

Cadmium is a highly toxic heavy metal that has been classified as a category I human carcinogen by the International Agency for Research on Cancer (IARC).¹ Nowadays, cadmium contamination in the environment, as we all know, mainly comes from the emission from the industrial production of batteries, fertilizers, pigments, metallurgy and electroplates. In particular, in recent years, an increasing number of quantum dots containing cadmium have been synthesized and used widely in various laboratories. The pollution to the environment due to these quantum dots should not be neglected. However, the cadmium that enters the soil and water is absorbed by animals and plants, which can finally enter humans body through intake, thus posing a serious potential threat to human health.^2–4 Moreover, the long-term exposure to cadmium-containing environments for humans or animals not only impedes the normal deposition of calcium in the bones and causes osteoporosis, but also impairs the normal function of the immune, nervous, renal and reproductive systems.^5,6 Therefore, it is of great significance to develop a rapid and accurate method for the qualitative and quantitative detection of cadmium ions in environmental samples.

To date, the primary means for detecting cadmium ions are atomic absorption spectrometry (AAS),^7,8 inductively coupled plasma mass spectrometry (ICP-MS),^9,10 electrochemical methods^11–14 and microfluidic technologies.¹⁵ These detection methods have the advantages of high sensitivity, low detection limit and wide linear range. However, these methods have high maintenance costs and complicated sample pretreatment methods. In this regard, a fluorescent probe/sensor is a promising alternative route for metal ion and anion detection due to their advantages of an excellent selectivity, high sensitivity, low cost, handy operation and biological compatibility compared with other detection methods. Therefore, the fluorescent probe/sensor has attracted increasing attention for its design and development.^16–22 Although cadmium ion fluorescent probes/sensors have been reported in literature,^23–28 there are still some difficulties that need to be overcome such as the complicated synthesis procedure, poor water solubility and slow response speed.

In the present work, a simply synthesized, highly sensitive and fast responsive turn-on fluorescence probe for Cd²⁺ based on a photo-induced electron transfer (PET) and CH=N isomerization was successfully synthesized. The fluorescence characterization of the obtained probe towards metal ions was investigated in detail. The synthetic probe exhibited an evident fluorescence enhancement and quick response to Cd²⁺. Meanwhile, the binding stoichiometry and binding mechanism were also explored through the Job's Plot method, FT-IR characterization and ¹H NMR titration experiment.

Experimental section

Materials and apparatus

All reagents and solvents employed for the synthesis were purchased from commercial suppliers and used without further purification. ADMPA was dissolved in methanol at a concentration of 2 mM as a stock solution and stored in the refrigerator at 4 °C. All of the metal ions, including Ag⁺, Al³⁺, Ba²⁺, Be²⁺, Ca²⁺, Cd²⁺, Co²⁺, Cr³⁺, Cu²⁺, Fe²⁺, Fe³⁺, Hg²⁺, K⁺, Mg²⁺, Mn²⁺, Ni²⁺, Pb²⁺ and Zn²⁺ were in the chloride form or nitrate form. All the other reagents and solvents used in the synthesis and analysis were of analytical reagent grade and distilled water was used throughout the experiment.

¹H NMR and ¹³C NMR experiments were conducted on a Varian AS 400 MHz NMR system in DMSO-d₆ with TMS as an internal standard. Elemental analyses for C, H and N were performed on a Vario EL III Organic Element Analyzer (Elementar, GER). The emission spectra were recorded on a FL-4600 spectrometer (Hitachi, Japan). The time-resolved fluorescence lifetime measurement was measured using a FLS920 transient fluorescence spectrometer (EI, UK). The FT-IR spectra were measured in the 4000–400 cm⁻¹ range with KBr pellets on a Spectrum Two FT-IR spectrometer (PerkinElmer, USA). The mass spectral data for DMP-CHO was measured on a LCQ DECA XP MAX mass spectrometer (Thermo, USA).

Synthesis

Synthesis of 1,10-phenanthroline-2,9-dicarboxaldehyde (DMP-CHO). As shown in Scheme 1, 10-phenanthroline-2,9-dicarboxaldehyde (DMP-CHO) was synthesized via a mildly modified synthetic procedure that was previously reported.²⁹ Briefly, selenium dioxide (2.77 g, 0.025 mol) was dissolved in 75 mL of 1,4-dioxane and heated to reflux for 30 min. Afterwards, 2,9-dimethyl-1,10-phenanthroline (2.61 g, 0.012 mmol) in 1,4-dioxane (50 mL) was added dropwise into the flask and the reaction was stirred at 85 °C for 6 h. The solid–liquid mixture was subsequently filtered by diatomite while hot. A pale-yellow precipitate formed in the filtrate after cooling to ambient temperature. After removing the solvent, the obtained crude product was further purified by recrystallization in DMF and washed with hot water several times. Yield: 85%. ¹H NMR (400 MHz, DMSO-d₆): 10.32 (s, 2H), 8.77 (d, 2H), 8.27 (d, 2H), 8.25 (s, 2H). ESI-mass m/z calculated value for 237.06; experimental value was 237.05.

Scheme 1 The synthetic route for the ADMPA probe.

Synthesis of 2,2′-((1E,1′E)-((1,10-phenanthroline-2,9-diyl)bis(methanylylidene)) bis(azanylylidene))diphenol (ADMPA). ADMPA was synthesized based on a reported procedure³⁰ with a slight modification. Generally, 0.1180 g (0.5 mmol) of 1,10-phenanthroline-2,9-di-carbaldehyde and 10 mL of methanol were successively added into a 50 mL round-bottom flask. The mixture was stirred at room temperature for 30 min. Then, 0.1310 g (1.2 mmol) of an o-aminophenol methanol solution (15 mL) was slowly added dropwise into the flask. The solution started to become turbid 5 min later. The reaction process was monitored by TLC. The reaction was stopped when the spot for the raw material disappeared. After removing the solvent, an ADMPA yellow product was obtained and washed with methanol for three times. Yield: 85%. ¹H NMR (400 MHz, DMSO-d₆): δ (ppm) 9.29 (s, 2H), 9.13 (s, 2H), 8.80 (d, 2H), 8.65 (d, 2H), 8.13 (s, 2H), 7.46 (s, 2H), 7.17 (t, 2H), 6.97 (d, 2H), 6.91 (t, 2H). ¹³C NMR (100 MHz, DMSO-d₆): δ (ppm) 159.58, 155.20, 152.27, 145.58, 137.47, 136.98, 130.05, 129.16, 128.15, 121.37, 120.43, 120.15, 116.96. Anal. calcd. for C₂₆H₁₈N₄O₂: C 74.63, H 4.34, N 13.39, found: C 74.26, H 3.96, N 13.68.

Fluorescence characterization

Procedures for fluorescence measurement. Firstly, 8.4 mg of ADMPA was dissolved in 10 mL of methanol to prepare a 2 mM ADMPA stock solution for subsequent use. The concentration of all the metal ion stock solutions was 10 mM. The test solution was obtained by adding 25 μL of the ADMPA stock solution and a fixed volume of each metal ion stock solution in a 10 mL disposable plastic centrifuge tube, and then diluting the solution to 10 mL via the addition of a DMF-water (v/v, 3 : 7) solution. After mixing the fluorescent probe with metal ions properly, the fluorescence spectra of ADMPA was obtained on a FL-4600 spectrometer at room temperature. The excitation wavelength of the fluorescence spectrometer was set at 275 nm. The λ_ex slit width was 2.5 nm, except for the quantitative determination experiments (5 nm). The λ_em slit width was 5 nm and the voltage was 700 V.

Job plot method. For the Job's plot measurement, ADMPA (2 mM) in methanol and Cd²⁺ (1 mM) in water were prepared. A series of test solutions that contained different mole ratios of Cd²⁺ and ADMPA were prepared and measured. The Job's plot was drawn by plotting F versus the mole fraction of Cd²⁺, where F was the fluorescence intensity of each test solution at 377 nm.

FT-IR characterization

For the FT-IR characterization, ADMPA powder, cadmium nitrate powder and potassium bromide powder were thoroughly mixed and ground at a mass ratio of 1 : 0.5 : 100, 1 : 1 : 100, and 1 : 2 : 100. After grinding, these powders were dried in a vacuum oven at 50 °C overnight. Finally, ADMPA and the different mass ratios of ADMPA-Cd²⁺ were measured using a Spectrum Two FT-IR spectrometer (PerkinElmer, USA).

¹H NMR titration experiments

For the ¹H NMR titration experiments, 0, 0.25, 0.5 and 1.0 equiv. of Cd²⁺ were added into the ADMPA (4.18 mg, 0.01 mmol) DMSO-d₆ solution to obtain different mole fractions of the ADMPA-Cd²⁺ complex. Then, the ¹H NMR spectra of these solutions were recorded on a Varian 400 MHz NMR system.

Recovery investigation

Tap water, Yanhu water and Zhujiang River (Guangzhou, China) water were filtered with a 0.22 μm microporous membrane and then mixed with DMF at a volume ratio of 7 : 3. Owing to the low concentration level of Cd²⁺ in the real sample, the addition recovery method was adopted. The concentration of ADMPA and Cd²⁺ were fixed to 5 μM and 1 μM, respectively.

Results and discussion

Selectivity of the ADMPA probe towards Cd²⁺

The selectivity of the fluorescent probe is very important for the selective determination of target analytes. For this purpose, the recognition ability of the ADMPA probe was investigated via the fluorescence characterization towards metal ions, including Ag⁺, Al³⁺, Ba²⁺, Be²⁺, Ca²⁺, Cd²⁺, Co²⁺, Cr³⁺, Cu²⁺, Fe²⁺, Fe³⁺, Hg²⁺, K⁺, Mg²⁺, Mn²⁺, Ni²⁺, Pb²⁺ and Zn²⁺. As shown in Fig. 1, the ADMPA probe presented a fluorescence emission at 377 nm under an excitation wavelength of 275 nm. However, only Cd²⁺ ion produced a noticeable turn-on fluorescence response to ADMPA (Fig. 1, black line) when the same amount of ADMPA was added into above mentioned cations. This was attributed to the formation of a ADMPA-Cd²⁺ complex with a more rigid conjugated structure, which hindered the rotation of CH=N and the suppression of the photo-induced electron transfer (PET) quenching process that resulted in the chelation-enhanced fluorescence (CHEF) effect.^31,32

Fig. 1 Emission spectral changes for ADMPA (5 μM) induced by the addition of various metal ions (5 μM) in a DMF-water (v/v, 3 : 7) solution at room temperature.

In order to investigate if the probe/sensor could be applied to detect target analytes in complicated environmental samples, the selectivity and anti-interference capability were critical factors used to evaluate the properties of the chemosensor. Thus, to further investigate the anti-interference capability of the ADMPA probe towards Cd²⁺ against 18 other competitive metal ions, binding competition experiments were performed. It can be seen from Fig. 2 that the fluorescence emission intensity changes for ADMPA in a DMF-water (v/v, 3 : 7) solution upon the addition of 2 equiv. of the other single metal ions (black bars). The results showed that most of the competing ions had no influence on the emission spectra of ADMPA apart from Cu²⁺ and Pb²⁺, which produced a certain fluorescence quenching after continuous treatment with 1 equiv. of Cd²⁺. This was attributed to the fact that Cu²⁺ and Pb²⁺ were in the same group of elements as Cd²⁺, which possessed the same charge and a similar electronic layer structure. Nevertheless, the quenching degree of the fluorescence intensity at 377 nm was relatively acceptable when the 18 mixed metal ions were added into the ADMPA-Cd²⁺ complex (Fig. S5†). All of the above mentioned phenomena strongly certified that ADMPA could be used as a highly selective and sensitive fluorescent ‘turn-on’ chemosensor for Cd²⁺.

Fig. 2 The influence of single metal ions (10 μM) on the interaction between ADMPA (5 μM) and Cd²⁺ (5 μM) in a DMF-water (v/v, 3 : 7) solution.

The fluorescence lifetime of the fluorescent molecular probe was closely related to its own structure and the micro-environment of the fluorescent probe. Researchers can directly understand the changes in the studied system by measuring the time-resolved fluorescence spectrum. Herein, the time-resolved fluorescence spectra of ADMPA and ADMPA-Cd²⁺ were measured. As shown in Fig. 3, the lifetime decays fit a single exponential decay profile with a lifetime of 3.78 ns (χ² = 1.031) and 3.31 ns (χ² = 1.072) for ADMPA and the ADMPA-Cd²⁺ complex, respectively, which were attributed to the inhibition of CH=N isomerization and the rearrangement of the charge rendered by the suppression of the photo-induced electron transfer (PET).

Fig. 3 Time-resolved fluorescence decay of ADMPA and ADMPA-Cd²⁺ in a DMF-water (v/v, 3 : 7) solution.

The response time for the probe towards the target is also an essential criteria for judging its performance, which determines whether it is suitable for on-site or/and on-line detection. Therefore, the response time for ADMPA towards Cd²⁺ was investigated by monitoring the fluorescence intensity change of ADMPA after the addition of Cd²⁺ on a FL-4600 fluorescence spectrophotometer. As shown in Fig. 4, it was clear that the fluorescence intensity of ADMPA sharply increased after the addition of an equal amount of Cd²⁺. Then, the signal remained roughly unchanged. It is worthwhile to note that the quick response time of ADMPA towards Cd²⁺ was only 6.6 s, which was 3–40 times faster than the reported cadmium ion probe (Table 1). The fast response time allows for the possibility of the on-site or on-time detection of Cd²⁺.

Fig. 4 On-line monitoring of the reaction velocity process for ADMPA (20 μM) and Cd²⁺ (20 μM) in a DMF-water (v/v, 3 : 7) solution.

Table 1 Comparison of the detection limit, detection media and response time for Cd²⁺ between the proposed method and those reported in literature

Probe	Solvent	λex/λem	LOD (μM)	Response time	Ref.
	DMSO/Tris–HCl (1 : 9)	620/639	—	20 s	23
	HEPES	418/611 653	0.032	4 min	24
	DMSO/H2O (1 : 1)	341/473	0.314	—	25
	ACN	365/418	0.128	—	26
	ACN/HEPES (1 : 5)	400/561	0.010	0.3 min	27
	MeOH/H2O (3 : 7)	400/485 560	—	—	33
	H2O	460/567	0.01	—	34
	HEPES	315/419	0.60	—	35
	ACN	402/519	0.32	—	36
	ACN/HEPES (2 : 8)	308/590	0.010	—	37
	MeOH/H2O (1 : 4)	360/470 585	0.27	—	38
	DMSO/HEPES (1 : 4)	460/511	0.077	—	39
	DMF/H2O (3 : 7)	275/377	0.029	6.6 s	This work

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Quantitative determination of Cd²⁺

The fluorescence sensing properties of ADMPA towards Cd²⁺ were also explored by fluorescence titration experiments in a DMF-water (v/v, 3 : 7) solution. As shown in Fig. 5, the fluorescence intensity of ADMPA (5 μM) at 377 nm gradually increased with the increase in the Cd²⁺ concentration. A good linear relationship (R² = 0.9960) between the fluorescence spectra of ADMPA (5 μM) and the Cd²⁺ concentration was obtained from 0.025 to 2.5 μM with a detection limit of 29.3 nM according to the equation, DL = 3SD/S, where SD is the standard deviation of ten times the blank measurements and S is the slope of the calibration curve. The detection limit of ADMPA towards Cd²⁺ in this work was much lower than most values reported in literature (Table 1). Furthermore, the response time of ADMPA towards Cd²⁺ (6.6 s) was also much faster than reported literature (Table 1). Finally, the ADMPA probe was applied for the detection of Cd²⁺ in tap water, lake water and Zhujiang River water. Satisfactory recoveries between 97.72% and 101.1% with a RSD (n = 3) under 4.8% were obtained, which suggested that the proposed method possessed a good accuracy (Table 2 and S2†).

Fig. 5 The linear relationship between the fluorescence intensity of ADMPA and the concentration of Cd²⁺. Inset: fluorescence wavelength scanning of the ADMPA solution (5 μM) upon the addition of various equivalents of Cd²⁺.

Table 2 The recovery of Cd²⁺ in Tap water/Yanhu water/Zhujiang River water

Real sample	Added/μM	Detected/μM	Recovery/%	RSD/% (n = 3)
Tap water	1.0	1.004	100.4	3.7
Yanhu water	1.0	1.011	101.1	2.6
Zhujiang River water	1.0	0.9772	97.72	4.8

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Binding affinity of ADMPA to Cd²⁺

In order to explore the binding mode for the coordination of ADMPA and Cd²⁺, a Job's plot analysis was performed. As shown in Fig. 6, it was clear that the fluorescence intensity reached a maximum value when the molar ratio of [Cd²⁺]/[Cd²⁺] + [ADMPA] was about 0.33. This clearly suggested that the ratio of ADMPA bound with Cd²⁺ was 2 : 1. The binding constant (K_a) for the ADMPA-Cd²⁺ complex was calculated with fluorescence titration spectra from Fig. 5 using a revised Benesi–Hildebrand equation:^40,41 I₀/I − I₀ = (a/b − a)(1/K_a[C] + 1), where I₀ and I are the fluorescence intensities of ADMPA at 377 nm in the absence and existence of Cd²⁺, respectively, a and b are constants, K_a is the association constant and [C] is the concentration of Cd²⁺.⁴² The calculated binding constant was 3.15 × 10⁵ M⁻¹ from the plot of 1/(I − I₀) against 1/[C] for Cd²⁺.

Fig. 6 Job's plot for determining the stoichiometry of ADMPA bound with Cd²⁺ in a DMF-water (v/v, 3 : 7) solution. The total concentration was 10 μM.

To further elucidate the mechanism for the binding modes between the ADMPA probe and Cd²⁺, FT-IR titration experiments for ADMPA towards Cd(NO₃)₂ were also performed. The infrared spectra for the ADMPA probe and different mass ratios of the ADMPA-Cd(NO₃)₂ complex were obtained. As shown in Fig. 7, there were apparent changes in the IR spectra between free ADMPA and ADMPA-Cd(NO₃)₂. Three distinct peaks for ADMPA at 3212 cm⁻¹, 1619 cm⁻¹ and 1251 cm⁻¹ represented a O–H vibration, CH=N vibration and C–O vibration, respectively. Those peaks gradually disappeared in the spectrum of ADMPA-Cd(NO₃)₂ with the increase in the Cd(NO₃)₂ mass. Meanwhile, new peaks at 2397 cm⁻¹, 1764 cm⁻¹, 1383 cm⁻¹ and 825 cm⁻¹ appeared. Among them, the peak at 1764 cm⁻¹ was obviously the stretching vibration peak of C=O and the other three peaks were the characteristic peaks of CO₂, NO₃⁻ and NO₂⁻, respectively. These changes clearly implied that the phenolic hydroxyl and imine may have been involved in the complex reaction between ADMPA and Cd²⁺. Owing to the electron transfer from ADMPA to Cd²⁺, the double bond in C=O was formed and the fluorescence was enhanced.

Fig. 7 FT-IR of ADMPA and the ADMPA-Cd²⁺ complex.

For a better understanding, ¹H NMR titration experiments were performed by adding various amounts of Cd²⁺ to ADMPA in DMSO-d₆. As shown in Fig. 8, the peak for OH₅ at 9.29 ppm gradually disappeared with the increase of Cd²⁺, while the signal for other hydrogen atoms at a different downfield shift changed from 0.04 ppm to 1.54 ppm (Table 3). These changes suggested that OH₅ participated in the coordination of ADMPA towards Cd²⁺ and the formation of the complex changed the conjugated structure of ADMPA, causing a shift in the hydrogen atoms of the benzene ring and phenanthroline skeleton.

Fig. 8 ¹H NMR titration of ADMPA and ADMPA with Cd²⁺ (DMSO-d₆) for (a) ADMPA only, (b) ADMPA with 0.25 equiv. of Cd²⁺, (c) ADMPA with 0.5 equiv. of Cd²⁺ and (d) ADMPA with 1.0 equiv. of Cd²⁺.

Table 3 ¹H NMR shifts for ADMPA and ADMPA with Cd²⁺

H atom	ADMPA	ADMPA + Cd^2+ (mole ratio 2 : 1)	Downfield shift
H1	8.62	8.30	0.32
H2	8.80	8.76	0.04
H3	8.13	7.93	0.20
H4	9.13	8.56	0.57
H5	9.29	Disappear	—
H6	6.97	5.59	1.38
H7	7.17	6.27	0.90
H8	6.91	5.72	1.19
H9	7.46	5.92	1.54

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Based on the above results, a possible binding mode for ADMPA towards Cd²⁺ was proposed, as shown in Fig. 9. In the absence of Cd²⁺, the nitrogen atom of imine transferred an electron to the phenanthroline ring (PET ON) and the C=N group could rotate freely. When Cd²⁺ was added, the phenolic hydroxyl and neighboring imine nitrogen reacted with Cd²⁺, resulting in the restriction of the PET process and C=N isomerisation. Meanwhile, there was a full rearrangement in the charge distribution of the entire molecule, which increased the rigidity and turned the fluorescence on.

Fig. 9 Proposed binding mode for ADMPA with Cd²⁺.

Conclusions

In conclusion, a highly-sensitive and fast responsive turn-on fluorescence ADMPA probe for Cd²⁺ detection was successfully developed based on 2,9-dimethyl- 1,10-phenanthroline and o-aminophenol. The binding mode of ADMPA and Cd²⁺ was confirmed by a Job's plot analysis and ¹H NMR titration experiments with a ratio of 2 : 1. The obtained ADMPA probe was applied for the detection of Cd²⁺ at a concentration as low as 29.3 nM in DMF-water. The proposed method was applied to detect Cd²⁺ in an actual sample with a satisfactory recovery. More importantly, the excellent response velocity provided a possibility for the real-time detection of Cd²⁺. Thus, the proposed method could be a promising alternative route for Cd²⁺ detection in environmental samples.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported and sponsored by the National Natural Science Foundation of China (21677053 and 21876033).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9ra06356k

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