Benzothiazole based multi-analyte sensor for selective sensing of Zn²⁺ and Cd²⁺ and subsequent sensing of inorganic phosphates (Pi) in mixed aqueous medium

Abstract

A rationally designed simple benzothiazole based Schiff base probe (L) exhibits distinct selective “turn-on” emission responses in the presence of Zn²⁺ and Cd²⁺ ions. Emission wavelengths over 600 nm were achieved with both the metal ions, which makes the system more viable for the study in biological complex medium. Further “turn-off” fluorometric response for the inorganic phosphates was achieved with both the L–Zn²⁺ and L–Cd²⁺ ensembles. The entire sensing process is perceptible to the naked eye and is viable in mixed buffered medium at physiological pH.

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Introduction

Detection of metal ions by exploitation of fluorescence-based techniques has been on the rise owing to the omnipresence of the same in biology, the environment and various fundamental processes such as osmotic regulation, catalysis, metabolism, biomineralization, and signaling.¹ Among the metal ions, selective and sensitive detection of Zn²⁺ ions is an intriguing task due to their spectroscopically silent 3d¹⁰ configuration. Detection of Cd²⁺ ion, which possesses very similar spectroscopic properties as Zn²⁺, is also highly preferred owing to the high toxicity and carcinogenic behaviour.^2–4 In the human body, Zn²⁺ ion is second most abundant transition metal ion after iron and plays key roles in various biological processes including enzymatic catalysis, stabilization of protein structures and modulation of interactions between macromolecules.^5–7 In mammalian cells, Zn²⁺ is estimated to be present in the range of 100 to 500 μM; the largest fraction being involved with metalloproteins.^8,9 Again, the disruption of Zn²⁺ homeostasis has been found to be associated with many neurological disorders such as Alzheimer's disease, Parkinson's disease, epilepsy and amyotrophic lateral sclerosis.^10,11

Among the various available techniques for sensing metal ions, fluorescence based sensing is quite preferable due to its high sensitivity, fast response time and the ease of handling. It enable high-contrast imaging of organs, soft tissues etc. and can unveil various metal related issues in bio-medical applications, environment. The first few generations of Zn²⁺ sensors were excited by UV light, which lead to severe photobleaching and photo-damages of the cells. Thus, fluorescent chemosensors with higher wavelength emission (lower energy) are required as they can overcome the auto fluorescence from various biological entities and can also minimize the photo-damages. Therefore, currently enormous efforts have been made in the synthesis of long range emissive fluorescent probes for metal ions. The NIR emissive (near infra-red) fluorescent chemosensors can penetrate very deep into the cells without much damage to them. Unfortunately, most of the existing NIR probes are derived from cyanine dyes, which suffer from poor stability issues.^12–14 Again, the use of metal-chemosensor ensembles for targeting anionic species is an increasing popular technique in the sensing arena.^15–17 The articulate behaviour of zinc chemosensor ensembles as subsequent sensors towards various anions,^18–20 amino acids²¹ and a number of small molecules has come up as a topic of interest.

Among the series of anions, special interest lies in the sensing of inorganic phosphate (Pi) comprising of H₂PO₄⁻, HPO₄²⁻, PO₄,³ considering their imperative roles in biological systems.^22–24 The effects of modern day pollution on the environment at the behest of the Pi family of anions cannot also be ignored. The utility of phosphate in bone mineralization, signal transduction and energy storage marks its unprecedented importance.²⁵ Its adverse effects have resulted in the phenomenon of algal blooming (eutrophication). Also, it leads to a marked decrease in the amount of dissolved oxygen in water, causing serious problems for the aquatic ecosystem.²⁶ In addition, it is also a potential threat to the renal and cardiovascular systems where its effects can incur vascular calcification and bone resorption.²⁷ The importance of this class of anions is also highly augmented by the fact that the serum phosphate level can even envisage the early mortality in HIV-positive adults.²⁸

Comparative difficulty in sensing Zn²⁺ in intricate biological systems to other transition metal ions arises due to the fact that Zn²⁺ ions are silent to common analytical techniques like Mossbauer, NMR, and electron paramagnetic resonance (EPR). Some success has been achieved so far for detection of Zn²⁺ by means of fluorescent chemosensing.^29–36

The number of highly proficient sensors is vast but most of them often hinge on the premise of rigorous multistep organic synthesis and purification which ultimately prolongs the time required and the cost incurred. Keeping these points in mind, we have herein developed a simple Schiff base chemosensor for Zn²⁺ and Cd²⁺ emitting in the long wavelength visible region. This is another step in our constant endeavour for designing benzothiazole based chemosensors.^37–40 A quinoline unit was hereby attached to a benzothiazole core through an imine bond to develop the final chemosensor. The quinoline moiety not only acts as fluorophore but can also bind to metal ions. The imine functionality was expected to continue the π conjugation which was necessary to obtain an output response at higher wavelength. The sp² hybridized imine N is also a well-known metal binder. In addition, the presence of both hard/borderline N and soft S in the benzothiazole core was expected to respond to multiple ions.

Experimental

Materials and methods

All materials for synthesis were purchased from commercial suppliers. The absorption spectra were recorded on a Perkin-Elmer Lamda-25 UV-vis Spectrophotometer using 10 mm path length quartz cuvettes in the range 250–700 nm wavelengths, while the fluorescence measurements were carried on a Horiba Fluoromax-4 Spectrofluorometer using 10 mm path length quartz cuvettes with a slit width of 4 nm at 298 K. The mass spectrum of L was obtained using Waters Q-ToF Premier Mass Spectrometer. The NMR spectra were recorded on a Varian FT-400 MHz instrument and the chemical shifts were presented in parts per million (ppm) on the scale. The following abbreviations were used to describe spin multiplicities in ¹H NMR spectra: s = singlet; d = doublet; t = triplet; m = multiplet.

Synthetic procedures

The synthetic scheme of the probe (L) is shown in Scheme 1. Condensation of 2-quinolinecarboxaldehyde (1.02 mmol) and 2-hydrazinylbenzothiazole (1.5 mmol) in methanol gave a pale yellow precipitate (L). It was then filtered and washed with cold ethanol give pure L. The structure of the L is fully confirmed by NMR, mass and single crystal X-ray diffraction.

Scheme 1 Synthesis of the ligand L.

Yield = 75%; ¹H NMR (400 MHz, DMSO, TMS): 12.83 (s, 1H), 8.44–8.41 (d, 1H), 8.28 (s, 1H), 8.08–7.99 (m, 3H), 7.81–7.77 (t, 2H), 7.65–7.61 (t, 2H), 7.36–7.32 (t, 1H), 7.19–7.15 (t, 1H). ¹³C: 147.75, 136.98, 130.41, 129.12, 128.32, 122.42. ESI-MS: m/z calculated for C₁₇H₁₂N₄S [M] = 304.08, found [M + H⁺] = 305.0835.

Crystallography

The intensity data were collected using a Bruker SMART APEXII CCD diffractometer, equipped with a fine focus 1.75 kW sealed tube Mo-Kα radiation (λ = 0.71073 Å) at 298(2) K, with increasing ω (width of 0.3° per frame) at a scan speed of 3 s per frame. The SMART software was used for data acquisition. Data integration and reduction were undertaken with SAINT and XPREP⁴¹ software. Multi-scan empirical absorption corrections were applied to the data using the program SADABS.⁴² Structures were solved by direct methods using SHELXS-97 and were refined by full-matrix least squares on F² using SHELXL-97 program package.^43,44 In the crystal structure, non-hydrogen atoms were refined anisotropically. Hydrogen atoms attached to all carbon atoms were geometrically fixed. Structural illustrations have been generated using MERCURY 1.3 for Windows.⁴⁵

UV-vis and fluorescence spectral studies

Stock solutions of various ions (50 × 10⁻³ mol L⁻¹) are prepared in deionized water. Chloride or nitrate salts are used for metal ions while tetrabutyl, tetraethyl or sodium salts of the corresponding anions are used for the preparation of anion stock solutions. The stock solution of L (5 × 10⁻³ mol L⁻¹) was prepared in DMSO. All studies are performed in 4 : 1 EtOH/HEPES buffer medium (5 mM, pH 7.4). For the titration experiments, each time a 1 × 10⁻³ M solution of L in a quartz optical cell of 1 cm optical path length was titrated with the escalating concentration of stock solutions by using a micropipette.

Evaluation of the apparent binding constant

Receptor L with an effective concentration of 10.0 × 10⁻⁶ M is used for the emission titration studies with Zn²⁺ solution. The effective Zn²⁺ and Cd²⁺ concentrations are varied between 0 and 60 μM for this titration.

The apparent binding constants for the formation of the respective complexes are evaluated using the Benesi–Hildebrand (B–H) plot (eqn (1)).^46,47

1/(I − I₀) = 1/{K(I_max − I₀)C} + 1/(I_max − I₀) (1)

I₀ is the emission intensity of L at λ = 612 nm for Zn²⁺ and 602 nm for Cd²⁺, I is the observed emission intensity at the particular wavelength in the presence of a certain concentration of the metal ion (C), I_max is the maximum emission intensity value that was obtained at λ = 612 nm and 602 nm for Zn²⁺ and Cd²⁺ respectively during titration with varying metal ion concentration, K is the apparent binding constant (M⁻¹) and was determined from the slope of the linear plot, and C is the concentration of the Zn²⁺ and Cd²⁺ added during titration studies.

Detection limit

The detection limit was calculated on the basis of the fluorescence titration. The fluorescence emission spectrum of L was measured 10 times, and the standard deviation of blank measurement was achieved. To gain the slope, the ratio of the fluorescence emission at 612 nm and 602 nm were plotted as a concentration of Zn²⁺ and Cd²⁺ respectively.

Consequently, the detection limit was calculated by making use of the following equation

Detection limit = 3σ/k (2)

where, σ is the standard deviation of blank measurement, and k is the slope between the ratio of fluorescence emission versus respective analyte concentration.

Results and discussion

The probe L was synthesized in good yield and in the pure form by a single step reaction (Scheme 1). The rationale in designing such a probe to sense multi-analytes is based on the following specific attributes: (1) the probe must consist of a binding unit (receptor) and a signaling unit (chromophore/fluorophore); (2) the probe should contain both cation and anion binding abilities in order to expand its sensing horizon and (3) it must also possess extensive conjugation in order to make long wavelength emission possible. Substantial UV-visible and fluorescence spectroscopy studies were performed to obtain the selectivity and sensitivity responses of L towards the various tested cations and anions.

Crystal structure of L

Block shaped crystals were grown from ethanolic solutions of the probe. The probe L was found to crystallize in P21 space group with Z = 2. The molecule was almost planar with a dihedral angle C11–C10–N2–N3 = 178.20. An intramolecular hydrogen bond between quinoline N and NH proton N3H⋯N1 (1.953 Å, 134.86) was also realized. This forced the receptor molecule to adopt a cis-like configuration around the imine bond (Fig. 1). The probability of both benzothiazole N atom and S atom in acting as hydrogen bond acceptors cannot be ignored considering their average bond distance of 2.8095 Å.

Fig. 1 (a) Crystal structure and (b) packing diagram of L.

CCDC no. 1496954, empirical formula: C₁₇H₁₂N₄S, M_w: 304.37, T = 298(2) K, monoclinic, space group: P21, a = 8.3771(9) Å, b = 6.2358(8) Å, c = 14.5374(13) Å, β = 99.659(9)°, V = 748.64(14) ų, Z = 2, D_x (g cm³) = 1.350, F (000) = 316, reflections collected/unique = 3820/1631[R_int = 0.0223], R₁ = 0.0604, wR₂ = 0.1258[I > 2sigma(I)], R₁ = 0.1047, wR₂ = 0.1546 (all data), GOF (F²) = 1.022.

UV-vis spectroscopic studies

The free probe L has an absorption maximum at 365 nm in 4 : 1 ethanol/HEPES buffer (pH 7.4, 5 mM) (Fig. 2 and S5, ESI†). This peak may be ascribed to an intramolecular π–π* charge transfer (CT) transition. The receptor was treated with various metal ions including Cr³⁺, Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Hg²⁺, Pb²⁺, Al³⁺, Ca²⁺, Ba²⁺, Ag⁺. It was exciting to observe that among the tested metal ions, Cu²⁺, Ag⁺, Zn²⁺, Cd²⁺, Hg²⁺, Co²⁺, Ni²⁺ and Pb²⁺ induced certain characteristic changes in the absorbance spectra of L. In most of the cases in which changes are induced, the intensity of the peak at 365 nm gets diminished to different extents for various metal ions.

Fig. 2 (a) Changes in the absorption spectra of L upon the addition of Zn²⁺ and Cd²⁺ ions in mixed buffer medium at room temperature. Inset: visual colour change upon the addition of Zn²⁺ and Cd²⁺ to L; (b) UV-visible titration spectra of L (10 μM) with varying concentrations of Zn²⁺.

Simultaneously, new peaks in the region from 480 nm to 500 nm were observed depending on the individual metals. However, for Ag⁺ ion it was seen that the new peak emerged at around 395 nm. The change in the spectral nature of L on interaction with all other metals was not prominent enough (Fig. 2 and S5, ESI†). An interesting range of colour changes were observed starting with green for Ag⁺ moving to different shades of orange for most other interacting metal ions and pink for Cu²⁺. It was exciting to note that a comparatively conspicuous blue shift was obtained upon addition of Ag⁺ in the UV-visible spectrum (Fig. S5, ESI†). This and the other remarkable colorimetric responses of L in the presence of Cu²⁺, Ag⁺, Zn²⁺, Cd²⁺, Hg²⁺, Co²⁺, Ni²⁺ and Pb²⁺ might be attributed to the ligand to metal charge transfer (LMCT) after formation of individual metal–L complexes for the respective metal ions.

However, considering our impetus on the sensing ability of L as an efficient fluorescent probe, we did not delve further into the colorimetric sensing ability of the probe and have just had a preview of the colorimetric behaviour of L in the presence of different metal ions. We have, rather, directly concentrated our focus to explore the fluorescence sensing ability of L in detail.

Fluorescence spectroscopic studies

We have studied the sensing property of the probe in detail with fluorescence emission spectroscopy. The probe showed a weak emission at 558 nm when excited at 490 nm. However, unlike the UV-vis spectra, the probe L displayed specific emission change only with Zn²⁺ and Cd²⁺ ions of all the tested metal ions. Addition of both the ions enhanced the fluorescence emission at 612 nm for Zn²⁺ and at 602 nm for Cd²⁺ while the intensity of the 558 nm peak remained almost constant (Fig. 3a). This, therefore, allowed for a window of 10 nm in the fluorescence emission signals of both these ions, possessing exceptionally similar properties, which is sufficient for their distinction. Stokes' shift of the order of around 54 nm and 44 nm for Zn²⁺ and Cd²⁺ ions was observed respectively. A 31.5 fold enhancement in fluorescence intensity was observed upon interaction of L with Zn²⁺ ions at 612 nm. This intensity enhancement was furthered to 36 times for Cd²⁺ ions at 602 nm wavelength (Fig. 3a). However, the utility of the ligand is restricted as the sensing becomes difficult when both the metal ions coexist in solution.

Fig. 3 (a) Fluorescence spectra of L (10 μM) in the presence of an excess (10 equivalents) of various metal ions in mixed buffer medium λ_ex = 490 nm, slit = 4/4 nm (b) changes in emission spectra of L (10 μM) with the incremental addition of Zn²⁺. Inset: intensity at 612 nm vs. [Zn²⁺] plot.

The metal ions selectivity profile of the probe was also studied to further understand the reason for these interesting red shifted fluorescence intensity enhancements, subsequent titrations with Zn²⁺ ions (Fig. 3b) and Cd²⁺ ions (Fig. S7, ESI†) were performed. An increasing Zn²⁺ ion concentration manifested a linear enhancement in fluorescence intensity at 612 nm. A plot of fluorescence intensity at 612 nm as a function of equivalents of added Zn²⁺ ions was also done. From the plot it was revealed that upon addition of around 0.5 equivalents of Zn²⁺ ions there was a sharp increase in the fluorescence intensity. On further additions this enhancement was halted and a decrease in fluorescence intensity was observed on subsequent additions (Fig. 3b, inset). This indicated towards the formation of a new fluorescence active compound. Similar observations were noted for the fluorescent titration study with Cd²⁺ ions as well. The steep increase for the plot of fluorescence intensity at 602 nm as a function of added equivalents of Cd²⁺ ions until 0.5 equivalents typified towards the realization of fluorescence active complex which may be similar to that formed by Zn²⁺ ions.

The next important question highlighting on the utility of the probe was its sensitivity. The Benesi–Hildebrand equation gave binding constant for Zn²⁺ ion. It was found to be 1.11 × 10⁴ M⁻¹ as calculated by using the fluorescence titration reading (Fig. S9, ESI†). The calculated lowest detection limit for Zn²⁺ was 2.23 × 10⁻⁶ M (Fig. S10, ESI†). The binding constant for Cd²⁺ ion was evaluated as 1.42 × 10⁴ M⁻¹ as calculated from the Benesi–Hildebrand equation using the fluorescence titration reading (Fig. S12, ESI†). The calculated lowest detection limit for Cd²⁺ was found to be 2.98 × 10⁻⁶ M (Fig. S13, ESI†). The linear response for Zn²⁺ was found be to in the range 5 × 10⁻⁶ M to 2.5 × 10⁻⁵ M. For Cd²⁺, the linearity range was between 5 × 10⁻⁶ M and 3 × 10⁻⁵ M (Fig. S28, ESI†). The pH dependence studies also elucidate the applicability of the probe for selective sensing in the pH range 4–10 (Fig. S25, ESI†).

Anion sensing with the metal – chemosensor ensemble

Although, the naked chemosensor possessed some anion binding sites, it was found to be insensitive to any of the anions in the aforementioned experimental medium. This may probably be due to the strong solvation effects of the anions by the solvent mixture. Therefore, the in situ formed L–Zn²⁺ ensemble was chosen for the anion sensing purpose as the positively charged metal center possessed a natural propensity for interaction with anions. To this end, both the L–Zn²⁺ and L–Cd²⁺ ensembles were treated with various anions such as F⁻, Cl⁻, Br⁻, I⁻, CO₃²⁻, HCO₃⁻, CH₃CO₂⁻ (OAc⁻), NO₃⁻, SO₄²⁻, PO₄³⁻ and the resulting change was monitored through UV-vis and fluorescence emission spectroscopy. Interestingly, among the aforesaid anions both the ensembles were found to be highly selective for the PO₄³⁻ anion. In both the cases, the orange colour of the solution was completely bleached. Addition of 1.0 equivalent of PO₄³⁻ anion (10 μM) solution resulted in complete quenching of the initial fluorescence emission of both the ensembles. It was exciting to note that the original fluorescence spectrum of the sole chemosensor was restored upon addition of the concerned anion. This suggested towards the sequestration of Zn²⁺ and Cd²⁺ ions by PO₄³⁻ anions. To validate the assertion, subsequent titrations of the Zn²⁺–L and Cd²⁺–L ensembles were carried out with PO₄³⁻ anions in the EtOH/HEPES (4 : 1) mixed aqueous medium. A continuous decrease in the emission intensity at 612 nm was observed for the L–Zn²⁺ ensemble and the resultant solution was devoid of any comparable fluorescence (Fig. 4b). Also, a plot of intensity at 612 nm of the L–Zn²⁺ ensemble against equivalents of PO₄³⁻ displayed a sharp fall in the emission intensity until PO₄³⁻ equivalents close to 0.5. The emission intensity remained steady for further equivalents thereon. The Job's plot obtained from the titration experiment suggests a 2 : 1 binding of PO₄³⁻ with metal ensemble (Fig. S17, ESI†) and the calculated binding constant is 2.0 × 10⁴ M⁻¹ (B–H equation, Fig. S18, ESI†). The LOD of the zinc ensemble for PO₄³⁻ anion is 1.26 × 10⁻⁷ M (Fig. S19, ESI†). We subsequently tested for the change in fluorescence intensity by titration of PO₄³⁻ with the L–Cd²⁺ ensemble as well (Fig. S21, ESI†). As previously encountered, constant fall in the emission intensity was observed at 602 nm for the L–Cd²⁺ ensemble. The resulting solution was, again, devoid of any noticeable fluorescence. The Job's plot obtained from the titration experiment suggests a 2 : 1 binding of PO₄³⁻ with Cd²⁺–L ensemble (Fig. S22, ESI†). The calculated binding constant was 3.33 × 10⁴ M⁻¹ (B–H equation, Fig. S23, ESI†). The LOD of the Cd-ensemble was found to be 1.75 × 10⁻⁷ M (Fig. S22, ESI†). The linear response for Zn²⁺–PO₄³⁻ ensemble was found to be between 1 × 10⁻⁵ M to 5 × 10⁻⁵ M. For Cd²⁺, the linearity range was between 1 × 10⁻⁵ M and 7 × 10⁻⁵ M (Fig. S28, ESI†).

Fig. 4 (a) Fluorescence spectra of L–Zn²⁺ ensemble in the presence of PO₄³⁻. λ_ex = 490 nm, slit = 4/4 nm; (b) changes in emission spectra of L–Zn²⁺ ensemble with incremental addition of PO₄³⁻.

To further validate our conjecture, we observed the UV-vis spectral pattern change upon Pi addition to the metal–L ensembles. As expected, the absorption maximum at 365 nm for naked L is restored and the characteristic 485 nm peak for the metal–L ensemble gets diminished after addition of PO₄³⁻ anion (Fig. S16 and S20, ESI†). This change was also perceptible to the naked eye as the orange coloured solution turns colourless.

However, neither any visual colour change nor any regeneration of the characteristic ligand peak at around 365 nm was observed in the UV-visible absorbance spectra after addition of any of the anions viz. F⁻, Cl⁻, Br⁻, I⁻, CO₃²⁻, HCO₃⁻, CH₃CO₂⁻ (OAc⁻), NO₃⁻, SO₄²⁻ even at higher mole ratios. This indicates the high selectivity of the L–metal ensembles for the phosphate family of ions. Therefore, these can be thus employed both as colorimetric as well as fluorescence based sensors. A comparison of fluorescent properties of some of the recent probes against L has also been provided (Table S1, ESI†).

Detection of Zn²⁺ and Cd²⁺ in real samples

The applicability of L in sensing Zn²⁺ and Cd²⁺ were also examined using available real water samples of complex nature. Tap, lake and river water were collected from laboratory, the serpentine lake in IIT Guwahati, and Brahmaputra river (near IIT Guwahati campus, Assam, India), respectively. Zn²⁺ and Cd²⁺ (7–21 μM) were spiked into these samples before the addition of L (10 μM). The changes in fluorescence intensity at 612 nm and 602 nm were recorded for Zn²⁺ and Cd²⁺ respectively (Fig. S27, ESI†). The presence of the metal ions in these water samples was determined from the distinctive fluorescence responses at 612 and 602 nm respectively.

Plausible mechanism of sensing

The weak fluorescence emission of the free receptor may be attributed to the intramolecular photoinduced electron transfer (PET) from the Schiff base N lone pair to the benzothiazole moiety. In addition, the C=N isomerization as a decay process of excited states may also render the chemosensor non-fluorescent.⁴⁸ However, the interaction of L with the target metal ions engaged the lone pair involved in PET, thereby suppressing the radiationless deactivation channel. At the same time, the free rotation around the imine bond is also restricted due to metal chelation resulting in a flat platform for better conjugation to take place. Amalgamation of these factors leads to an enhancement of the fluorescence intensity with bathochromic shifts for the metal ions in concern (Scheme 2)

Scheme 2 Schematic representation of multi-analyte sensing by chemosensor L.

The nature of the complex thus formed was further confirmed by both Job's plot (Fig. S8 and S11, ESI†) and mass spectrometry (Fig. S14 and S15 ESI†). That it is a case of formation of 2 : 1 (host : guest) complexes with both Zn²⁺ and Cd²⁺ ions is confirmed by both the techniques. Mass spectral analysis supported the 2 : 1 complex formation between L and the Zn²⁺ ion via a molecular ion peak at m/z = 671.1090 ([2L + Zn²⁺]) (Fig. S14, ESI†). Similar mass spectral pattern was also obtained for Cd²⁺ with the molecular ion peak at 722.5367 ([2L + Cd²⁺]) (Fig. S15, ESI†).

Persisting with the same, we also performed subsequent UV-vis titrations with Zn²⁺ ions (Fig. 2b) and Cd²⁺ ions (Fig. S6, ESI†) to maximize on the binding behaviour in the ground state. As stated earlier, the free probe L has an absorption maximum at 365 nm. Upon incremental addition of Zn²⁺ ions, isobestic points at 330 nm and 422 nm were observed in the UV-visible spectra.

When Cd²⁺ ions were added sequentially, isobestic points at 326 nm and 418 nm were observed. These observations suggests towards the formation of a new UV-active complex through an electron rearrangement of L in the presence of Zn²⁺ and Cd²⁺ ions.

In persistence with the same, the ‘turn-off’ fluorescence behaviour of the L–Zn²⁺ and L–Cd²⁺ ensembles could be explained by considering the strong binding affinity of PO₄³⁻ towards these metal ions. The sequestration of the concerned metal ions by the P-containing anions in concern and formation of stable Zn–PO₄³⁻ and Cd–PO₄³⁻ adducts respectively release the free probe in the solution, which renders the strong emission of the metal–L ensembles.

These changes are also perceptible to the naked eye as the solution changes from colourless to orange and back to colourless upon addition of Zn²⁺, Cd²⁺ ions and P-containing anions respectively.

Conclusion

We have rationally designed and synthesized a simple fluorogenic probe, which displays specific responses with Zn²⁺ and Cd²⁺ ions in mixed buffer medium at physiological pH. We could also able to distinguish the two congeners by turn-on emission in different wavelength. In addition, the L–Zn²⁺ and L–Cd²⁺ ensembles were tested for subsequent selectivity for the Pi anions in the same medium.

Acknowledgements

G. D. acknowledges CSIR (01/2727/13/EMR-II) and Science & Engineering Research Board (SR/S1/OC-62/2011) New Delhi, India for financial support and CIF IITG for providing instrument facilities. RS and AG acknowledge IIT Guwahati for fellowship.

Notes and references

1. E. L. Que, D. W. Domailli and C. J. Chang, Chem. Rev., 2008, 108, 1517–1549.
2. R. R. Lauwerys, A. M. Bernard, H. A. Reels and J. P. Buchet, Clin. Chem., 1994, 40, 1391.
3. M. Waisberg, P. Joseph, B. Hale and D. Beyersmann, Toxicology, 2003, 192, 95–117.
4. S. Ellairaja, R. Manikandan, M. T. Vijayan, S. Rajagopal and V. S. Vasantha, RSC Adv., 2015, 5, 63287–63295.
5. C. J. Frederickson, J. Y. Koh and A. I. Bush, Nat. Rev. Neurosci., 2005, 6, 449–462.
6. J. M. Berg and Y. Shi, Science, 1996, 271, 1081–1085.
7. B. L. Vallee and D. S. Auld, Biochemistry, 1990, 29, 5647–5659.
8. G. Danscher and M. Stoltenberg, J. Histochem. Cytochem., 2005, 53, 141–153.
9. C. J. Chang, J. Jaworski, E. M. Nolan, M. Sheng and S. J. Lippard, Proc. Natl. Acad. Sci. U. S. A., 2004, 101, 1129–1134.
10. M. Kawahara, D. Mizuno, H. Koyama, K. Konoha, S. Ohkawara and Y. Sadakane, Metallomics, 2014, 6, 209–219.
11. M. W. Bourassa and L. M. Miller, Metallomics, 2012, 4, 721–738.
12. Z. Guo, W. Zhu, M. Zhu, X. Wu and H. Tian, Chem.–Eur. J., 2010, 16, 14424–14432.
13. J. J. Lee, S. A. Lee, H. Kim, L. Nguyen, I. Noh and C. Kim, RSC Adv., 2015, 5, 41905–41913.
14. P. S. Yao, Z. Liu, J. Z. Ge, Y. Chen and Q. Y. Cao, Dalton Trans., 2015, 44, 7470–7476.
15. R. M. Duke, E. B. Veale, F. M. Pfeffer, P. E. Kruger and T. Gunnlaugsson, Chem. Soc. Rev., 2010, 39, 3936–3953.
16. J. L. Sessler, P. A. Gale and W. S. Cho, Anion Receptor Chemistry, RSC Publishing, Cambridge, UK, 2006.
17. H. T. Ngo, X. Liu and K. A. Jolliffe, Chem. Soc. Rev., 2012, 41, 4928–4965.
18. D. Karak, S. Das, S. Lohar, A. Banerjee, A. Sahana, I. Hauli, S. K. Mukhopadhyay, D. A. Safin, M. G. Babashkina, M. Bolte, Y. Garcia and D. Das, Dalton Trans., 2013, 42, 6708–6715.
19. Z. Dong, X. Le, P. Zhou, C. Dong and J. Ma, New J. Chem., 2014, 38, 1802–1808.
20. V. Luxami, K. Paul and I. H. Jeong, Dalton Trans., 2013, 42, 3783–3786.
21. S. Kaur, V. Bhalla and M. Kumar, Chem. Commun., 2014, 50, 9725–9728.
22. J. Hatai, S. Pal and S. Bandyopadhyay, Tetrahedron Lett., 2012, 53, 4357–4360.
23. T. Schenk, N. M. G. M. Appels, E. D. A. van, H. Irth, U. R. Tjaden and D. G. J. van, Anal. Biochem., 2003, 316, 118–126.
24. C. P. Mathews and K. E. van Hold, Biochemistry, The Benjamin/Cummings Publishing Company, Inc, Redwood City, CA, 1990.
25. J. Hirose, H. Fujiwara, T. Magarifuchi, Y. Iguti, H. Iwamoto, S. Kominami and K. Hiromi, Biochim. Biophys. Acta, 1996, 1296, 103–111.
26. S. E. Manahan, Fundamentals of Environmental Chemistry, CRC Press, Taylor & Francis Group, LLC, 3rd edn, 2008.
27. S. H. Hong, S. J. Park, S. Lee, S. Kim and M. H. Cho, J. Toxicol. Sci., 2015, 40, 55–69.
28. D. C. Heimburger, J. R. Koethe, C. Nyirenda, C. Bosire, J. M. Chiasera, M. Blevins, A. J. Munoz, B. E. Shepherd, D. Potter, I. Zulu, A. Chisembele-Taylor, B. H. Chi, J. S. A. Stringer and E. K. Kabagambe, PLoS One, 2010, 5, e10687.
29. P. Nyren, Anal. Biochem., 1987, 167, 235–238.
30. S. Xu, M. He, H. Yu, X. Cai, X. Tan, B. Lu and B. Shu, Anal. Biochem., 2001, 299, 188–193.
31. L. Nyiranshuti, D. P. Kennedy, A. G. DiPasquale, A. L. Rheingold and R. P. Planalp, Polyhedron, 2016, 109, 147–153.
32. Y. Mikata, R. Ohnishi, A. Ugai, H. Konno, Y. Nakata, I. Hamagamie and S. Sato, Dalton Trans., 2016, 45, 7250–7257.
33. A. Jiménez-Sánchez, B. Ortíz, V. O. Navarrete, N. Farfánc and R. Santillan, Analyst, 2015, 140, 6031–6039.
34. A. Visscher, S. Bachmann, C. Schnegelsberg, T. Teuteberg, R. A. Matac and D. Stalke, Dalton Trans., 2016, 45, 5689–5699.
35. C. E. Felton, L. P. Harding, J. E. Jones, B. M. Kariuki, S. J. A. Pope and C. R. Rice, Chem. Commun., 2008, 6185.
36. G. K. Tsikalas, P. Lazarou, E. Klontzas, S. A. Pergantis, I. Spanopoulos, P. N. Trikalitis, G. E. Froudakis and H. E. Katerinopoulos, RSC Adv., 2014, 4, 693.
37. A. Gogoi, S. Samanta and G. Das, Sens. Actuators, B, 2014, 202, 788–794.
38. A. Gogoi and G. Das, RSC Adv., 2014, 4, 55689.
39. A. Gogoi, S. Mukherjee, A. Ramesh and G. Das, RSC Adv., 2015, 5, 63634.
40. A. Gogoi, S. Mukherjee, A. Ramesh and G. Das, Anal. Chem., 2015, 87, 6974–6979.
41. Saint, Smart and XPREP, Siemens Analytical X-ray Instruments Inc., Madison, Wisconsin, USA, 1995.
42. G. M. Sheldrick, SADABS, software for Empirical Absorption Correction, University of Gottingen, Institute fur Anorganische Chemieder Universitat, Tammanstrasse 4, D-3400 Gottingen, Germany, 1999–2003.
43. G. M. Sheldrick, SHELXS-97, University of Gottingen, Germany, 1997.
44. G. M. Sheldrick, SHELXL-97, Program for Crystal Structure Refinement, University of Gottingen, Germany, 1997.
45. Mercury 1.3 Supplied with Cambridge Structural Database, CCDC, Cambridge, U.K., 2003–2004.
46. H. A. Benesi and J. H. Hildebrand, J. Am. Chem. Soc., 1949, 71, 2703–2707.
47. F. Han, Y. Bao, Z. Yang, T. M. Fyles, J. Zhao, X. Peng, J. Fan, Y. Wu and S. Sun, Chem.–Eur. J., 2007, 13, 2880–2892.
48. J. Wu, W. Liu, J. Ge, H. Zhang and P. Wang, Chem. Soc. Rev., 2011, 40, 3483–3495.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1496954. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra22840b

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