First-in-class hydrazone–pyrazoline sensors for selective detection of Zn²⁺, Cd²⁺, and Hg²⁺ in aqueous environments

Abstract

Six first-in-class hydrazone–pyrazoline fluorescent sensors that selectively detect Zn²⁺, Cd²⁺, and Hg²⁺ at three distinct wavelengths in aqueous environments are reported. These novel hybrid sensors are synthesized in just two steps from inexpensive, commercially available materials, enabling rapid generation and screening for desirable photophysical properties. Lead sensor P1 was validated for in situ detection of group 12 metals in river and pond water using a portable, low-cost device. Crucially, this system enabled rapid, naked-eye detection of toxic metals directly in the environment without the need for expensive instrumentation or mains electricity. These findings establish a new direction in pyrazoline sensor research and set a high benchmark for future sensor development.

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Introduction

The first three metals in group 12 of the periodic table are zinc, the second most abundant transition metal in the human body critical to life,¹ cadmium, a highly toxic industrial pollutant linked to numerous cancers,² and mercury, with well-established neurotoxicity.³ The European Union (EU) maximum drinking water limits for cadmium and mercury are 44 nM and 5 nM respectively.⁴ The World Health Organisation (WHO) has a recommended zinc drinking water limit of 46 μM.⁵ The regular monitoring of group 12 metals in aqueous environments such as drinking water and external water courses is paramount. The ability to distinguish between Zn²⁺, Cd²⁺, and Hg²⁺ in aqueous environments is a major challenge requiring expensive and complex equipment not widely available, for example, ICP-MS. Fluorescence spectroscopy offers several advantages⁶ over traditional techniques including high selectivity, low limits of detection (LoD), and rapid analysis. Fluorescent sensors are “turned on” when the analyte increases fluorescence emission (λ_em)⁷ or “turned off” when the analyte decreases λ_em.⁸ Inhibition of photoinduced electron transfer (PET)⁹ or the chelation enhancement effect (CHEF)¹⁰ can increase λ_em. Decreased λ_em may result from fluorescence resonance energy transfer (FRET)¹¹ or chelation-enhanced quenching effect (CHEQ).¹² While numerous fluorescent sensors have been reported for Zn²⁺ and Cd²⁺,¹³ few can differentiate between these metals at different λ_em.¹⁴ The pyrazoline scaffold¹⁵ (blue in Fig. 1) is highly advantageous due to its unique fluorescence properties¹⁶ and modular structure from readily accessible chalcone precursors.¹⁷ Chalcones, while utilized as precursors for pyrazolines, have also been reported to display useful sensing properties.¹⁸ Pyridine is a well-established metal chelator often integrated into fluorescent sensors for Zn²⁺, Hg²⁺ and Fe³⁺ detection.¹⁹ Simple pyrazoline A displayed a “turn on” response only in the presence of Zn²⁺ and Cd²⁺;²⁰B is a “turn on” sensor²¹ for Fe³⁺/Fe²⁺ whereas C displayed a “turn off” response in the presence of Fe³⁺/Fe²⁺.²¹ Pyrazole D (red in Fig. 1)²² demonstrated that an additional acetyl group greatly improved Zn²⁺/Cd²⁺ selectivity. A–D reveal that structural complexity is not essential for complex functionality^20–22 highlighting the potential of these scaffolds in sensor development. During a recent pyrazoline synthesis study (see ESI S1†) side product P1 was identified (Scheme 1), isolated, and discovered as a “turn on” sensor for Zn²⁺ at λ_em 560 nm and Cd²⁺ at 510 nm and a “turn off” sensor for Hg²⁺ at λ_em 460 nm. This unexpected side product is a highly attractive first-in-class hydrazone–pyrazoline multi-analyte group 12 sensor in aqueous environments.

Fig. 1 Pyrazoline and pyrazole sensors; images reproduced from ref. 20–22 with permission from RSC.

Scheme 1 Unexpected synthesis of side product P1 from C1.

Experimental

Chemicals and instruments

Chemicals, solvents and reagents were purchased from commercial sources and used without further purification. PE refers to petroleum ether, bp 40–60 °C. Spectroscopy was performed with CHROMASOLV® gradient grade acetonitrile for HPLC, ≥99.9%, from Sigma-Aldrich. The metal complexes used in this study were LiCl, NaCl, KCl, CaCl₂, MgCl₂, CuCl₂, NiCl₂, ZnCl₂, CdCl₂, RuCl₃, CoCl₂, MnCl₂, PbCl₂, ZnCl₂ and HgI₂.

UV/vis spectroscopy was performed on an Agilent Cary5000 using quartz cuvettes with a 1 cm pathlength using HPLC grade MeCN, in the 250–500 nm range with 0.2 s dwell time. Detector switchover occurred at 350 nm. FTIR spectroscopy was performed on a Bruker VERTEX 70 spectrometer. Fluorescence spectroscopy was performed on an Edinburgh Instruments FLS1000 with a xenon excitation source, 2 nm bandwidths for both excitation and emission monochromators, a scan speed of 1 nm and a dwell time of 0.2 s. Fluorescence quartz cuvettes with a 1 cm pathlength were used throughout with HPLC grade MeCN. A 100 watt 365 nm Analytik Jena high intensity UV lamp was used to image the sensors.

Synthesis of chalcones C1–C4

The following general procedure was followed (see the ESI† for more detailed information), using a method adapted from a previous synthesis,²² 6 mmol 2,6-acetylpyridine was added to a stirred solution of 3.0 mmol aldehyde in MeOH followed by the addition of 3.0 mmol NaOH and stirring was continued at room temperature. After 24 hours the precipitate was filtered, washed with copious amounts of cold H₂O collected and dried to afford the desired chalcone without further purification.

Synthesis of hydrazone–pyrazolines P1–P6

The following general procedure was followed: 4.0 mmol of 2-hydrazinopyridine was added to a stirred solution of 1.0 mmol of required chalcone in 50 mL MeOH and heated to 60 °C. After 24 hours, the solvent was removed under reduced pressure, 100 mL H₂O was added and it was extracted into 3 × 50 mL ethyl acetate. The ethyl acetate fractions were combined and the solvent was removed under reduced pressure to give an oil which was then purified by column chromatography with 6 : 4 ethyl acetate : petroleum ether to afford the desired product.

Results and discussion

Chalcones C1–C4 were prepared using a previous procedure²² in acceptable yields (13–60%). A direct chalcone to hydrazone–pyrazoline synthesis was developed to give novel compounds P1–P4 (Scheme 2) in good yield (26–61%).

Scheme 2 Synthesis of hydrazone–pyrazoline series P1–P4.

This two-step synthesis from inexpensive commercially available starting materials unlocks a new class of previously unexplored pyrazoline sensors. Furthermore, the modular nature of the pyrazoline scaffold enables a high degree of modification unlocking a deeper understanding of their sensing properties. With four novel hydrazone–pyrazolines in hand, we investigated their photophysical properties against the group 12 metals using well-established protocols.^22–24 UV/vis studies confirmed that P1 undergoes chelation with both Zn²⁺ and Cd²⁺ with the formation of a new band at 285 and 290 nm respectively (Fig. 2A and B). A ¹H NMR titration study for P3 in the presence of the group 12 metals indicated broadening and downfield movement for multiple signals (Fig. 2C for P3 with Hg²⁺). The broad singlet at 8.54 ppm attributed to the hydrazone proton H^a shifted to 9.31 ppm on the addition of 2.0 equivalents Hg²⁺. Significant shifts in the remaining protons were also observed; for example, the pyridine signal from H^b moved from 7.79 ppm to 7.99 pm and the aryl singlet for H^c moved from 6.54 to 6.68 ppm. This is a well-established observation with previous pyrazoline sensors undergoing chelation.^{20–23,24a,b}

Fig. 2 UV/vis study of P1 (20 μM, MeCN) with 0–20 equivalents of Zn²⁺ (panel A) and Cd²⁺ (panel B). ¹H NMR studies of P3 (9 μM, deuterated MeCN), (panel C) (i) P3 only and (ii) +2.0 equivalents of Hg²⁺.

The fluorescence response of P1 (φ_f 4.0%, τ 3.4 ns) was confirmed with the formation of new emission bands at 560 nm with Zn²⁺ (φ_f 1.0%, τ 14 ns) and 510 nm with Cd²⁺ (φ_f 1.0%, τ 13 ns) in 7 : 3 MeCN : H₂O (Fig. 3A). On addition of Hg²⁺ a “turn off” response at λ_em 460 nm (φ_f < 1.0% τ 4.0 ns) is observed. Of note is that the response is retained in an aqueous solution demonstrating the real-world application of this sensor. A similar fluorescence profile was obtained with P2–P4 suggesting that substitution on the aryl ring was well-tolerated a range of both electron donating and electron withdrawing substituents. To determine the importance of hydrazone, two further pyrazolines were synthesised; P5 using phenyl hydrazine and P6 with methyl hydrazine (Scheme 3). P5 displayed no distinguishing features suggesting that the pyridine ring in P1–P4 was critical for group 12 selectivity. P6 with methyl units displayed insignificant fluorescence on addition of the group 12 metals suggesting that an aromatic unit was required. The contrast between P1 and P5 is striking confirming that pyridine units are essential for group 12 selectivity (Fig. 4).

Fig. 3 Fluorescence studies of P1–P4 (A–D) 15 μM, 7 : 3 MeCN : H₂O at λ_ex 285 nm, on addition of 5.0 equivalents of the indicated metal; cps is counts per second. Cuvette images were obtained under the irradiation of a 100 W λ_ex 365 nm lamp.

Scheme 3 Synthesis of hydrazone–pyrazoline P5–P6.

Fig. 4 Fluorescence studies of P5 (A) and P6 (B) 15 μM, 7 : 3 MeCN : H₂O at λ_ex 285 nm; cps is counts per second. Cuvette images were obtained under the irradiation of a 100 W λ_ex 365 nm lamp.

P1 was selected as the lead compound and submitted for further investigation. A titration study confirmed λ_em at 560 nm (yellow emission) which peaked with 5.0 equivalents of Zn²⁺ (Fig. 5B). A Zn²⁺ limit of detection (LoD) of 6.4 μM was calculated which is well below the WHO recommended drinking water limit. A similar study with Cd²⁺ confirmed λ_em at 510 nm (green emission) which reached a plateau with 5.0 equivalents of Cd²⁺ (LoD 2.4 μM). The 50 nm difference in λ_em between Zn²⁺ and Cd²⁺ is significant with very few examples reporting such a distinction between two group 12 metals.¹⁴ To our surprise Hg²⁺ generated an opposite response, a “turn off” response, with a decrease in λ_em at 460 nm, Hg²⁺ LoD 10.0 μM. These LoD values agree with those of previously reported pyrazoline sensors for Zn²⁺ and Cd²⁺ (ESI S6†). Competition assays were performed to determine the response of P1 in the presence of a range of interferents (Fig. 6 for P1 with Zn²⁺).

Fig. 5 (A) Fluorescence studies of P1 15 μM, 7 : 3 MeCN : H₂O at λ_ex 285 nm. (Panels B–D) Titration studies for Zn²⁺, Cd²⁺ and Hg²⁺, respectively; cps is counts per second. Cuvette images were obtained under the irradiation of a 100 W λ_ex 365 nm lamp.

Fig. 6 Competition assay for P1 with Zn²⁺ in the presence of a range of interferents. The white bar represents the sensor (20 μM, 7 : 3 MeCN : H₂O) with 5.0 equiv. of the indicated cation; the black bars are the same with 5.0 equiv. of Zn²⁺ after equilibrating for 2 min. λ_ex 285 nm with λ_em 560 nm; the red line indicates the “turn on” response with Zn²⁺ only, and cps is counts per second.

Several paramagnetic metals including Fe³⁺, Ru³⁺, Co²⁺, and noticeably Cu²⁺ disrupted the “turn on” response to Zn²⁺ at 560 nm (Fig. 6). This is a common phenomenon for pyrazolines with paramagnetic metals.^13b,c,22,23 Interestingly, the biological group 1 and 2 metals, Li⁺, K⁺, Na⁺ and Mg²⁺, resulted in very minor disruption suggesting that P1 could be used for the monitoring of Zn²⁺ in living systems high in these metals, an active research area.^13a,26 A similar profile was observed with Cd²⁺; paramagnetic metals Mn²⁺, Ru³⁺, Co²⁺ and Cu²⁺ hampered λ_em 510 nm but a good response remained with the group 1 and 2 metals (Fig. 7). A further assay of P1 with Hg²⁺ showed that paramagnetic metals disrupted the “turn off” response (Fig. 8). Under these conditions, the group 1 and 2 metals did not significantly influence the “turn off” response to Hg²⁺ suggesting that P1 could be a useful sensor for the “turn on” detection of Zn²⁺ at 560 nm and the “turn off” detection of Hg²⁺ at 460 nm in biological systems.

Fig. 7 Competition assay for P1 with Cd²⁺ in the presence of a range of interferents. The white bar represents the sensor (20 μM, 7 : 3 MeCN : H₂O) with 5.0 equiv. of the indicated cation; black bars are the same with 5.0 equiv. of Cd²⁺ after equilibrating for 2 min. λ_ex 285 nm with λ_em 500 nm.

Fig. 8 Competition assay for P1 with Hg²⁺ in the presence of a range of interferents. The white bar represents the sensor (20 μM, 7 : 3 MeCN : H₂O) with 5.0 equiv. of the indicated cation; the black bars are the same with 5.0 equiv. of Hg²⁺ after equilibrating for 2 min. λ_ex 285 nm with λ_em 460 nm; the red line indicates the “turn off” response with Hg²⁺ only.

To determine the real-world application of P1 in large-scale monitoring, several studies of a variety of water sources were conducted including drinking water (tap and mineral water see Fig. 9 and ESI S5†). A strong response to 100 μM Zn²⁺ in tap water (Fig. 9) with a visible colour change from blue to yellow was observed at λ_ex 365 nm (Fig. 9A inset). A similar study with Cd²⁺ in tap water produced a more noticeable response at 50 μM Cd²⁺ (Fig. 9B) and a green colour change under λ_ex 365 nm irradiation (Fig. 9B inset). The “turn off” response to Hg²⁺ was detectable in tap water (Fig. 9C) with a reduction in λ_em at 460 nm with both 50 μM and 100 μM Hg²⁺. Mineral water (see ESI S5†) yielded a comparable result. The visible colour change on the addition of Zn²⁺ (yellow), Cd²⁺ (green) and Hg²⁺ (blue) suggests that P1 could form a prototype for a low-cost and portable field-based device. A low-cost prototype was constructed consisting of a portable excitation source and a suitable dark box (ESI S8†).²⁵ Samples from two environmental locations (river and pond water) were taken and analysed in the laboratory and in situ using a portable device (Fig. 10 for pond water).

Fig. 9 Group 12 metal triggered fluorescence response of P1 (20 μM, 7 : 3 MeCN : H₂O) in tap water; Zn²⁺ (panel A), Cd²⁺ (panel B) and Hg²⁺ (panel C). Cuvette images under the irradiation of the 100 W λ_ex 365 nm lamp.

Fig. 10 Group 12 metal triggered fluorescence response of P1 (20 μM, 7 : 3 MeCN : H₂O) in pond water; +Zn²⁺ (panel A), +Cd²⁺ (panel B), +Hg²⁺ (panel C) and location (D).

At two different concentrations, 50 μM and 100 μM of Zn²⁺, Cd²⁺, and Hg²⁺, a fluorescence response was triggered analogous to the drinking water study. The decrease in λ_em with Zn²⁺ (Fig. 10A) was less than expected possibly due to additional interferents present in the environment, possibly sediment or bacteria (Fig. 10A). In the presence of Cd²⁺ the change in λ_em is noticeable and a visible change to green is observed (Fig. 10B inset). A decrease in λ_em at 460 nm was observed on addition of Hg²⁺ (Fig. 10C inset) confirming that P1 can distinguish Cd²⁺ from Hg²⁺ in the field that can be observed with the naked eye. A similar result was obtained in river water (Fig. 11). This study confirmed that P1 can be used outside of a laboratory environment with naked eye detection of the group 12 metals in both stagnant pond and free flowing river waters. To confirm the robustness of P1, a reversibility study was performed which confirmed that P1 can be used for multiple cycles for Zn²⁺ detection (Fig. 12A). The proposed binding mechanism of P1 with Zn²⁺ (Fig. 12B) is 1 : 1 and agrees with previous studies on A–D and the Job plot (ESI S7†).^20–23 While hydrazone units have been incorporated into multi-analyte fluorescent sensors previously,²⁷P1–P4 are the first reported sensors combining hydrazone and pyrazoline units which distinguish Zn²⁺, Cd²⁺, and Hg²⁺ at different λ_em in aqueous environments.

Fig. 11 Group 12 metal triggered fluorescence response of P1 (20 μM, 7 : 3 MeCN : H₂O) in river water; +Zn²⁺ (panel A), +Cd²⁺ (panel B), +Hg²⁺ (panel C) and location (D).

Fig. 12 A reversibility study for P1 in the presence of multiple cycles of Zn²⁺ and EDTA (panel A) and the proposed binding mode of P1 with Zn²⁺ (panel B).

Conclusions

Six first-in-class hydrazone–pyrazoline sensors were synthesised with P1–P4 displaying excellent selectivity towards group 12 metals. The aryl ring was well-tolerated to both electron donating and withdrawing substituents; however, substitution of pyridine for a phenyl unit in P5 abolished group 12 selectivity (Fig. 13). Substitution of the aryl ring on hydrazone and pyrazoline for methyl units in P6 significantly disrupted the fluorescence intensity. P1 was selected for further investigation and while a few heavy metals were significant interferents to the fluorescent response for P1, its ability to detect group 12 metals in real-world samples was confirmed. Combining P1 with a low-cost portable device enabled rapid visual detection by the naked eye of the 12 metals in rural environments without expensive apparatus or mains electricity. The group 1 and 2 metals exhibited very little influence on the fluorescence response indicating that P1 could also be a highly useful sensor for the selective detection of Zn²⁺ and Hg²⁺ in biological systems; for example cell culture and living systems.²⁶ This highly unexpected discovery establishes a new subset of pyrazoline sensors, hydrazone–pyrazoline hybrid sensors. These hybrid sensors composed of both hydrazone and pyrazoline functional units set a high benchmark for future group 12 sensors. Further work is underway to transform this discovery into a second generation of sensors that retain the group 12 metal selectivity of P1 but with improved properties (increased λ_em intensity and reduced interference). The high tolerance of the aryl R¹ group to substitution enables the attachment of these sensors to solid substrates as reported previously by Magri et al.²⁸ This provides valuable applications for enhanced sensor design and molecular logic gate development.²⁹ These are priority objectives for ongoing work and will be reported in due course.

Fig. 13 Summary of P1–P6; EDG is the electron donating group and EWG is the electron withdrawing group.

Data availability

The data supporting this article have been included as part of the ESI.†

Author contributions

Alexander Ciupa designed, synthesized, characterised, and performed all spectroscopy studies and authored the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The author acknowledges Steven Robinson for assistance with time-of-flight high resolution mass spectrometry and Krzysztof Pawlak for assistance with fluorescence spectroscopy. Emily Cunliffe is acknowledged for valuable discussions. This work made use of shared equipment located at the Materials Innovation Factory, created as part of the UK Research Partnership Innovation Fund (Research England) and co-funded by the Sir Henry Royce Institute.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ay00639b

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