Dual anion colorimetric and fluorometric sensing of arsenite and cyanide ions

Abstract

A naphthalene appended probe, 2-((2-hydroxynaphthalen-1-yl)methylene)hydrazine carbothioamide, was synthesized and found to recognize AsO₂⁻ and CN⁻ ions with turn-on emission fluorescence over different anions in a DMF : H₂O (HEPES buffer, pH = 7.2) (9 : 1, v/v solution) medium. The probe was characterized using different techniques including NMR, IR, CHNS, UV-visible and ESI mass spectroscopy. This probe shows colorimetric change and an enhancement in the fluorescence emission with arsenite and cyanide ions among the other anions. The 1 : 1 and 1 : 2 stoichiometries of the probe with arsenite and cyanide ions, respectively, were calculated from the Job's plots based on the UV-visible spectra. The binding constant was established using the B–H (Benesi–Hildebrand) plots for both anions arsenite and cyanide as 3.1 × 10⁵ and 1.9 × 10⁶, respectively. The limit of detection (LOD) of arsenite and cyanide ions was 66 nM and 77 nM, respectively, using the emission spectra. The binding affinity of probe L with both anions was determined using NMR, DFT optimization, ESI-mass spectroscopy, electrochemical behavior and optical studies. The probe is the first chemical sensor that detects both major toxic anions with significantly high detection limits.

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1. Introduction

Anions demonstrate their necessity in a broad range of chemical and biological processes. Various efforts have been devoted to generate numerous receptors for anionic species over the last two decades.¹ Among all the anions, it is well known that arsenic (in the form of anions) and cyanide ions are extremely toxic and lethal to living organisms. Arsenic is the 20^th most abundant element in the earth's crust. It mainly exists in the four oxidation states, As(V), As(III), As(0) and As(−3), in the environment.² Inorganic arsenic, viz. arsenite As(III) and arsenate As(V) is more toxic than organic arsenic, viz. monomethylarsonic acid and dimethylarsonic acid.³ Arsenite [As(III)] is more toxic than arsenate [As(V)] due to its binding with the sulfhydryl unit of proteins, which can intervene with the reactions of other enzymes and proteins.⁴ The binding of trivalent arsenic with specific proteins can convert the conformation and functionality of the protein as well as hamper their reaction with other proteins. As(V) can also interrupt the conversion of ATP to ADP via the permanent replacement of phosphate groups due to its resemblance to phosphite ions.⁵ The main source of arsenic exposure in the environment is from ground water or drinking water.^6,7 The chronic toxicity of arsenic has adverse effects on human health, such as skin cancer, skin lesions, neurotoxicity, cardiovascular diseases and diabetes.⁸ Similarly, cyanide anions are also detrimental to biological systems and the environment. Its acute poisoning can damage the body's central respiratory system because it can easily bind with heme-proteins to block cytochrome c oxidase, which hinders the electron transfer chain in the mitochondria.⁹ It is released in the environment by ammonia manufacturing, electroplating, steel production and extraction of gold.¹⁰ Inhalation of toxic cyanide can occur from absorption by the lungs and exposure via the skin, polluted drinking water and contaminated food.¹¹ As per as the guidelines of the World Health Organization (WHO) and U.S. Environment Protection Agency (USEPA), the permissible limits of arsenic and cyanide in drinking water are 0.01 ppm (or 10 ppb) and 0.2 ppm, respectively.¹² There have been many methods developed for the detection of arsenite and cyanide ions. Electrochemical analysis and ion chromatography are the traditional methods, which require time consuming procedures and the use of sophisticated instrumental techniques,¹³ whereas chemical sensors are another approach, which are simple, affordable and expeditious in real time monitoring. The sensing process frequently uses absorption and emission spectroscopy, which precisely monitor and sometimes detect using the naked eye.¹⁴ Chemical sensors for arsenite anions are rarely available with fluorescence intensity enhancement. Cyanide ions are analysed by their Lewis basicity, nucleophilicity¹⁵ and quality of making hydrogen bonds in a solution.¹⁶ There are numerous mechanisms, including internal charge transfer,¹⁷ single electron transfer,¹⁸ fluorescence resonance energy transfer¹⁹ and plated electrode,^20–22 which support the fluorescence behaviour for anions.

In this report, a single probe was developed that can sense both arsenite and cyanide ions. The sensing mechanism involves the probe having two acidic protons, which results in deprotonation via interaction of both anions as well as hydrogen bonding as revealed by NMR spectroscopy and detailed theoretical studies. To date, no reports are available based on the direct detection of both the toxic arsenite and cyanide anions together using a single chemical sensor with a high limit of detection limit in the range of μM to nM. Most of the previous reports on chemical sensors that detect multiple anions using the same probe molecule only detect common anions. The present protocol has the advantage that it detects those anions, which attract more interest from researchers due to their high toxicity and adverse effects i.e. AsO₂⁻ and CN⁻ with the same platform. The as-synthesized probe is a potential candidate to sense multiple anions whilst maintaining a high limit of detection using the same platform. Sensing using this probe is cost effective in terms of the reagents used for its synthesis as well as the reduced number of steps used in sensing because it can sense both the anions simultaneously.

2. Experimental section

2.1 Reagents and instrumentation

The sodium salts of the anions used were of analytical grade, purchased from Merck and used without further purification. 2-Hydroxy-1-naphthaldehyde and thiosemicarbazide were purchased from Sigma Aldrich (99%). All absorption spectra were recorded on a Shimadzu, UV-3600 double beam spectrophotometer using a square quartz cell with a path length of 10 mm. NMR spectra were recorded on a JEOL 400 MHz spectrophotometer. All emission spectra were recorded on a Horiba RF-5301PC using a standard quartz cell (path length, 3 cm). Vibrational spectra were recorded on a Perkin-Elmer FT-IR 1000 spectrophotometer. Elemental analysis (CHNS) was performed on an Elementar Model Vario El-III instrument. Cyclic voltammetry was carried out on an CHI760E electroanalyser with a glassy carbon electrode as the working electrode, Ag/AgCl as the reference electrode and platinum wire used as an auxiliary electrode with 0.1 M tetrabutylammonium perchlorate used as the supporting electrolyte at a scan rate of 0.1 V s⁻¹. The mass of the probe and the probe with the anions was analysed using a WATERS Q-TOF premier-HAB213 mass spectrometer. The fluorescence lifetime was recorded using a Horiba Jobin Yvon fluorocube fluorescence life time system.

2.2 Synthesis of the probe (L)

2-((2-Hydroxynaphthalen-1-yl)methylene)hydrazine carbothioamide (L). Thiosemicarbazide (1 mmol, 0.091 g) was dissolved in ethanol (10 mL) in a round bottom flask and 2-hydroxy-1-naphthaldehyde (1 mmol, 0.158 g) in ethanol (10 mL) was then added dropwise to the solution with stirring. After completion of the reaction, the reaction mixture was refluxed for 5 h according to a previously reported literature procedure.²³ A yellow coloured precipitate was obtained, which was filtered and recrystallized from DMF : ethanol (1 : 4) (Scheme 1).

Scheme 1 The synthesis of probe (L).

Yield. 80%. Calcd for C₁₂H₁₁N₃OS: C, 58.76; H, 4.52; N, 17.13; S, 13.07; O, 6.52 and found: C, 59.76; H, 3.22; N, 15.87; S, 12.97, O, 8.18. FT-IR data (ESI Fig. S1†) (KBr ν_max cm⁻¹): NH₂: 3254, N–H: 3165, C=S: 1394, C=N: 1611, O–H: 3447, UV-Visible (ESI Fig. S2†) (DMF : H₂O (9 : 1), λ_max nm): 331, 368 nm. ¹H NMR (DMSO, 400 MHz, δ/ppm) (ESI Fig. S3†): N–H: 11.36 (s, 1H), O–H: 10.51 (s, 1H), CH=N: 9.00 (s, 1H), NH₂: 8.48 (s, 1H), 8.16 (s, 1H), 7.83 (d, J = 9.0 Hz, 1H), 7.80 (d, J = 8.1 Hz, 2H), 7.53 (t, J = 7.5, 1H), 7.33 (t, J = 7.5 Hz, 1H), 7.15 (d, J = 9.0 Hz, 1H). ¹³C NMR (DMSO, 100 MHz, δ/ppm) (ESI Fig. S4†): 177.81, 157.16, 143.60, 133.04, 132.05, 129.23, 128.60, 128.44, 124.01, 123.44, 118.89, 110.29. ESI-mass of probe L m/z 246.0547, (M + H)⁺ (ESI Fig. S14†).

3. Results and discussion

3.1 Visualization test

The preliminary test for the detection of both anions was performed via a colorimetric test. Fig. 1 shows the performance of an equimolar concentration (5.0 × 10⁻⁵ M) of probe in DMF : H₂O (HEPES buffer, pH = 7.2) (9 : 1, v/v solution) with 5 equivalents of the anion solutions (1 mM). A sudden colour change was observed, which turned light yellow to dark yellow with arsenite and cyanide ions among the other anions. The colour change was observed due to deprotonation or strong hydrogen bonding between the probe and the anions (AsO₂⁻ and CN⁻). Moreover, the selectivity of arsenite and cyanide ions with probe was analyzed using absorption and emission spectra.

Fig. 1 The colorimetric change of the probe with different anions in DMF : H₂O (HEPES buffer, pH = 7.2) (9 : 1, v/v solution).

3.2 UV-vis studies of the probe with different anions

The selectivity of the anions (AsO₂⁻ and CN⁻) with probe L was investigated using UV-Vis studies among different anions including Cl⁻, Br⁻, F⁻, I⁻, SCN⁻, H₂PO₄⁻, HPO₄²⁻, CH₃COO⁻, NO₃⁻, SO₄²⁻, SO₃²⁻, PO₄³⁻, IO₃⁻ and N₃⁻ in the DMF : H₂O (9 : 1, v/v solution) medium. The probe showed two absorption peaks at 332 nm (due to π–π* transitions) and 369 nm (due to n–π* transitions).

Arsenite and cyanide ions have the ability to abstract hydrogen or make hydrogen bonds with the probe. Upon the addition of all the anions investigated, a new absorption band appeared at 452 nm in the UV-vis spectra in the case of arsenite and cyanide ions while this peak was absent for the other anions. The selectivity of the probe for only these two anions over the others may be attributed to its dissociation energy i.e. pK_a value with these anions, which provide more stability to the probe. Fig. 2 represents the selectivity with AsO₂⁻ and CN⁻ ions among other anions. The same experiment was performed using fluorescence spectroscopy and the intensity enhancement was observed with AsO₂⁻ and CN⁻ among all the other anions studied. This study revealed that the probe was selective for arsenite and cyanide anions.

Fig. 2 The UV-visible spectra of probe L (20 μM solution) with different anions.

3.3 Binding sites and stoichiometry

The binding stoichiometry of the probe L with arsenite and cyanide ions was observed in the absorption spectra of equimolar solutions (50 μM) of the probe L upon a gradual variation in the mole fraction with the anions (AsO₂⁻ and CN⁻). Fig. 3(a) and (b) show the change in the absorption spectra with different mole fraction of anions in the Job's plot,^24,25 which support the 1 : 1 and 1 : 2 stoichiometry with arsenite and cyanide ions (50 μM) respectively. Using the Job's plot, the mole fraction value for arsenite ion was 0.55 and for cyanide ion it was 0.66, which endorse the 1 : 1 and 1 : 2 stoichiometry. The absorption spectra show the changes upon the addition of the anions to the probe (L); the bands at 332 nm and 369 nm were quenched with an increase in the absorption band at 452 nm due to interaction of the anions with the probe (L). The isosbestic point was revealed from the non-interacted to interacted probe with anions (ESI Fig. S5 and S6†). The formation constants were calculated and found to be 3.6 × 10⁵ and 5.8 × 10⁶ for the arsenite and cyanide ions, respectively, by the Job's continuous variation method.

Fig. 3 (a) The Job's plot obtained with an equivalent mole concentration of probe L (50 μM) and arsenite ion (50 μM). (b) The Job's plot obtained with an equivalent mole concentration of probe L (50 μM) and cyanide ion (50 μM).

3.4 Fluorescence emission spectra

Fluorescence studies were carried out in the same medium with a 10 μM concentration of the probe (L); Fig. 4 shows the selectivity test using the emission spectra. First, the selectivity test was performed with probe L (10 μM) and 2 equivalent of all anions (40 μM). At an excitation wavelength of 363 nm, a “turn-on” emission occurred in the case of arsenite and cyanide ions over all the other anions studied (Cl⁻, Br⁻, F⁻, I⁻, SCN⁻, H₂PO₄⁻, HPO₄²⁻, CH₃COO⁻, NO₃⁻, SO₄²⁻, SO₃²⁻, PO₄³⁻, IO₃⁻ and N₃⁻). The deprotonation and strong hydrogen bonding between the probe and both the anions (arsenite and cyanide) caused an enhancement in the emission. Hydrogen was deprotonated from the –OH group and hydrogen bonding with the –NH group occurred. Absorption and emission studies are strong evidence, which indicates that the probe is more selective for arsenite and cyanide ions among all the other anions studied.

Fig. 4 The fluorescence emission spectra of probe L (10 μM) with different anions in DMF : H₂O (9 : 1, v/v solution).

To confirm the sensitivity of L towards CN⁻ and AsO₂⁻, dual anion treatment was also performed with probe L. In favour of dual anion studies, equal concentrations (10 μM + 10 μM) of the anions were used with probe L. Fig. 5(a) and (b) reveals the interference effect of the secondary anion in both cases viz. AsO₂⁻ and CN⁻. Single anion (Cl⁻, Br⁻, F⁻, I⁻, SCN⁻, H₂PO₄⁻, HPO₄²⁻, CH₃COO⁻, NO₃⁻, SO₄²⁻, SO₃²⁻, PO₄³⁻, IO₃⁻ and N₃⁻) treatment of the probe is indicated by the red bars and the blue bars represent L + CN⁻ with the other anions in Fig. 5(a). In the case of L + AsO₂⁻ shown in Fig. 5(b), the blue bars represent the cases with interfering anions and the red bars the case of single anions with probe L. The interference studies revealed that there was no interference by the other anions in the case of both anions whereas AsO₂⁻ and CN⁻ act as interfering ions with each other. Furthermore, the interfering studies with different metal ions were also performed and supported there was no interference by other metal ions with the arsenite and cyanide ions (ESI Fig. S12 and S13†). Therefore, with the help of the interference studies it can be concluded that the probe was selective for AsO₂⁻ and CN⁻ anions.

Fig. 5 (a) The interference study of probe L with different anions in the presence of AsO₂⁻. The red bar indicates probe L + anions and the blue bar indicates probe L + anion + AsO₂⁻. (b) The interference study of probe L with foreign anions in the presence of CN⁻. The red bar shows the L + anions and the blue bar represents L + anion + CN⁻.

These findings were strongly supported by the UV-Vis analysis of the probe with the various ions mentioned.

3.5 Emission titration of probe L with AsO₂⁻ and CN⁻

An emission titration was performed with 2.5 mL of probe L (10 μM) and each anion (AsO₂⁻ and CN⁻) were added gradually to the same amount of probe in DMF : H₂O (HEPES buffer, pH = 7.2) (9 : 1, v/v solution). The association constant of both anions with the probe were calculated from fluorescence titration i.e. 3.1 × 10⁵ and 1.9 × 10⁶, respectively for the anions (AsO₂⁻ and CN⁻) using the Benesi–Hildebrand plot.²⁶ The B–H plot was constructed between 1/(I − I_o) and 1/[A⁻], where I is the emission intensity²⁷ of probe L, I_o for L + anions (AsO₂⁻ and CN⁻) and I_max is the emission intensity of complete binding of anions with L and A⁻ is the concentration of anions.

Fig. 6(a) and (b) represent the B–H plots of L + CN⁻ and L + AsO₂⁻, respectively. Similarly, the limit of detection (LOD) was calculated using the plot between (I − I_o)/(I_max − I_o) and log(anions). Fig. 7(a) and (b) are the plots for the LOD for AsO₂⁻ and CN⁻ with probe L i.e. 66 nM (6.6 × 10⁻⁸ M) and 77 nM (7.7 × 10⁻⁸ M), respectively in DMF : H₂O (9 : 1, HEPES buffer, pH = 7.2), which are significantly less than the permissible limit cited by the WHO. In a previous report a minimum detection limit of 0.18 μM to 2.1 μM for CN⁻ and 10.0 μM to 1.32 μM for AsO₂⁻ were observed^28–31 whereas, in this work the LOD for AsO₂⁻ and CN⁻ was 66 nM and 77 nM, respectively. Whereas in DMF : H₂O (9 : 1) the detection limit of arsenite and cyanide with probe L was 68 nM and 0.16 μM, respectively (ESI Fig. S17–S20† in DMF : H₂O). Table 1 presents a comparison of both solvent conditions such as DMF : H₂O and DMF : buffer.

Fig. 6 (a) A fluorescence emission titration of probe L with different concentrations of cyanide ion at an excitation wavelength of 363 nm in DMF : H₂O (HEPES buffer, pH = 7.2) (9 : 1, v/v solution). The inset shows the B–H plot obtained for cyanide ions. (b) The fluorescence emission titration of probe L with different concentrations of arsenite ion at an excitation wavelength of 363 nm in DMF : H₂O (HEPES buffer, pH = 7.2) (9 : 1, v/v solution). The inset represents the B–H plot obtained for arsenite ions.

Fig. 7 (a) The limit of detection (LOD) determined for AsO₂⁻ ions using the fluorescence emission spectra. (b) The limit of detection (LOD) determined for CN⁻ ions using the fluorescence emission spectra.

Table 1 The UV-vis and fluorescence wavelengths, LOD and binding constant in different solvent conditions

Sample	UV-study	Fluorescence study	LOD	Binding constant
Probe L	332, 369 nm	—	—	—
L + CN^− (DMF : buffer)	332, 452 nm	495 nm enhanced fluorescence	77 nM	1.9 × 10^6
L + CN^− (DMF : H2O)	332, 452 nm	495 nm enhanced fluorescence	0.16 μM	2.5 × 10^5
L + AsO2^− (DMF : buffer)	332, 452 nm	495 nm enhanced fluorescence	66 nM	3.1 × 10^5
L + AsO2^− (DMF : H2O)	332, 452 nm	495 nm enhanced fluorescence	68 nM	3.2 × 10^5

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3.6 pH studies

The performance of the probe L and the probe when treated with both anions was found to be strongly dependent on the pH of the medium because the probe was enriched with proton sensitive donar sites. It was essential to investigate the pH dependence of the probe and the probe with both anions. The pH study of probe L showed that at acidic pH the intensity did not change as much as at basic pH. On further increment in the pH, deprotonation occurred as well as an increase in the fluorescence intensity. The same experiment was performed with L + AsO₂⁻ and L + CN⁻, which exhibited that in case of L + AsO₂⁻ the compound was stable between pH 8–11 and after that the intensity decreased and the compound became turbid.

However, probe L with CN⁻ demonstrated that the compound was stable from pH 7–11 and beyond this pH range, the compound was found to degrade [Fig. 8(a) and (b)]. The pH studies of the probe and the probe with anions revealed that a pH range between 7 and 11 was suitable for sensing the anions.

Fig. 8 (a) The pH dependence study of probe L with arsenite ions. (b) The pH dependence study of probe L with cyanide ion.

3.7 Binding interactions

Further, the interactions of the probe with AsO₂⁻ and CN⁻ ions were demonstrated using an ¹H NMR titration study. Experiments of the probe L and with both anions were performed in DMSO. In the NMR spectrum of the probe L the peaks at δ 9.040 ppm, δ 11.378 ppm and δ 10.494 ppm were designated as the –CH proton, –NH proton and –OH proton, respectively and the other doublet and triplet corresponded to the aromatic protons. Upon the addition of the CN⁻ and AsO₂⁻ ions, the proton signals were shifted upfield in both cases.

In the case of AsO₂⁻, a substantial upfield shift (Δδ = 0.003 ppm) was observed due to the resonance of the –OH proton with π–e⁻ cloud of aromatic region whereby, the electronegativity on the molecule increased. The peak for the –OH proton (δ 10.494 ppm) was found to completely disappear and similarly, the peak for the –NH proton (11.378 ppm) decreased. On the other hand, the addition of CN⁻ ion to the probe lead to a significant upfield shift (Δδ = 0.019 ppm) and the peaks corresponding to –NH and –OH completely disappeared. Meanwhile, the aromatic signals were also shifted upfield. The ¹H NMR studies clearly illustrate that both anions interacted with the probe L via deprotonation of the –NH and –OH groups in the case of CN⁻ whereas, in the case of AsO₂⁻ deprotonation and hydrogen bonding occurred with the –OH and –NH protons, respectively. Fig. 9(a) and (b) show the binding interactions with and without the anions.

Fig. 9 (a) The ¹H-NMR spectra of probe L and probe L + CN⁻ showing the reliable interactions between probe L & CN⁻ ions. (b) The ¹H-NMR of probe L and probe L + AsO₂⁻ showing the interaction of probe L and AsO₂⁻ ions.

Further, the deprotonation behavior of probe L in the case of both anions (AsO₂⁻ and CN⁻) was confirmed using ESI-mass spectrometry (ESI Fig. S14–S16†). The mass (m/z) of the probe L was 246.0547 (M + H)⁺, which changed upon the addition of the anions to m/z = 244.05 (M − 2H + H)⁺ in the case of cyanide and with arsenite the m/z value was 245.0587 (M − H + H)⁺, which supported the 1 : 2 and 1 : 1 stoichiometry between anions and the probe L.

3.8 Theoretical studies

By theoretical perspective the host and guest interaction was verified using density functional theory (DFT) calculations. The optimized geometry of probe L and L + anions (AsO₂⁻ and CN⁻) was obtained in the gas phase using the Gaussian 09 W computational program.³² This was performed with B3LYP functions and basis sets 6-31G(d, p) for probe L and L + anions. In the HOMO, the electron density of probe L was spread over the thiosemicarbazone unit while in the LUMO, the electron density was localized on the naphthalene unit. However, the energy band gap calculated between the HOMO–LUMO of probe L was 0.07 eV. The electron density of probe L in the HOMO–LUMO was effected by a deprotonating agent like CN⁻ and AsO₂⁻. As these anions deprotonate the proton from the probe the electron density increased in the HOMO in the case of CN⁻. Similarly, in the case of AsO₂⁻, the electron density was more effected than CN⁻ due to influence of hydrogen bonding. As a result, the decrease in the energy band gap and optimized structures in both cases are shown in Fig. 10. In the case of L + CN⁻ and L + AsO₂⁻, the energy band gap was diminished up to 0.06 eV and 0.04 eV, respectively. The electron distribution in the case of L + 2CN⁻ in the HOMO was around the thiosemicarbazone unit while in the LUMO it was mostly distributed around the naphthalene unit. Similarly, for L + AsO₂⁻, the electron cloud was localized around the naphthalene unit in the HOMO and in the LUMO, the electron density was localised on the AsO₂⁻ unit, which supports the deprotonation and hydrogen bonding mechanism.³³

Fig. 10 The optimized structure determined by DFT calculations and the energy band gap between probe L and probe L with both anions (AsO₂⁻ & CN⁻).

3.9 Cyclic voltammetry studies

Furthermore, the electrochemical demeanour of probe L and L + anions (AsO₂⁻ & CN⁻) were analysed in DMF solution containing 0.1 M TBAP as the supporting electrolyte. The voltammogram of probe L presented one irreversible reduction peak at −1.65 V and one oxidation peak at 1.01 V. After the addition of an AsO₂⁻ solution to the probe there was not any change in the irreversible reduction peak at −1.65 V but the current dropped-off in comparison to the previous case and a new peak appeared at 0.36 V that supports As(0) was oxidised to As(III);³⁴ simultaneously, the peak at 1.01 V was diminished with an electrochemical difference of 0.07 V. On the other hand, the oxidation and reduction peaks at −1.65 V and 1.01 V were shifted up to −0.85 V and 1.06 V, respectively, which supports the formation of anionic species and an increased in the π-conjugation in the case of cyanide,^35,36 concurrently, a new oxidation peak appeared at 0.21 V that supports the interaction between the probe L and cyanide ion (ESI Fig. S7–S9†).

3.10 Lifetime decay measurement

In order to confirm the turn-on emission behavior of probe L with arsenite and cyanide, the lifetime was measured for probe L with both the anions (AsO₂⁻ & CN⁻). The lifetime follows the mono-exponential decay of probe L with arsenite ion and the lifetime was 0.98 ns. Similarly, probe L with cyanide ions also follows the mono-exponential decay, which depicts that the lifetime of the probe L with cyanide was 0.90 ns as the probe L with cyanide and arsenite ion shows turn-on emission at an excitation wavelength of 363 nm.³⁷ Fig. 11(a) and (b) present the plots of the lifetime decay curves.

Fig. 11 (a) The lifetime profile of probe L with arsenite ions. (b) The lifetime profile of probe L with CN⁻ ions.

4. Applications

4.1 Sensitivity test

To check an unknown concentration of both anions in water, the concentration of probe L was maintained at 10 μM, while the concentration of arsenite ion was varied from 0–20 μM. The fluorescence intensity was measured at 363 nm for all the solutions. Then, a calibration plot was plotted between the concentration and the change in the intensity resulting in a linear relationship. A similar experiment was performed with cyanide ions, which results in a similar linear plot (ESI Fig. S10–S11†). Using this plot, an unknown concentration of arsenite and cyanide ion can be determined by measuring the fluorescence intensity in water and also in the presence of all the other anions studied.

4.2 Real water analysis

For the assessment of the practical application of probe L, the probe was applied to analyse real water samples. The water sample used was tap water from Roorkee and known amounts of cyanide and arsenite were added into the water samples. The assay was calculated by spiking a known amount of standard cyanide and arsenite solutions followed by evaluating its recovery. The recovery of the different known amounts of cyanide added was acquired from 95% to 104%, whereas for arsenite ion, it was from 93% to 99%, which showed that the application probe L in real water samples was quite feasible. Tables 2 and 3 represent the study with real water sample analysis.

Table 2 The determination of cyanide ions in tap water samples using probe L

Sample	Added CN^− (M)	Found^a CN^− (M) ± SD	Recovery (%)
a Standard deviation calculation for three measurements.
Tap water	5.0 × 10^−5	5.2 ± 0.4 × 10^−5	104%
	10.0 × 10^−5	9.5 ± 0.2 × 10^−5	95%
	15.0 × 10^−5	14.8 ± 0.2 × 10^−5	98.6%

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Table 3 The determination of arsenite ions in tap water samples using probe L

Sample	Added AsO2^− (M)	Found^a AsO2^− (M) ± SD	Recovery (%)
a Standard deviation calculation for three measurements.
Tap water	5.0 × 10^−5	4.9 ± 0.1 × 10^−5	99%
	10.0 × 10^−5	9.3 ± 0.3 × 10^−5	93%
	15.0 × 10^−5	14.7 ± 0.3 × 10^−5	98%

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5. Conclusions

The probe L was synthesized and characterised using different techniques including UV-vis, FT-IR, ESI-mass and NMR spectroscopy. This probe displayed high selectivity as well as sensitivity towards both the toxic arsenite and cyanide anions when compared to other biologically admissible anions. Uniquely, this probe acts as a chemodosimeter for both ions (AsO₂⁻ and CN⁻) with LODs as low as ca. 66 nM for arsenite ions and 77 nM for cyanide ions. The binding affinity of probe L to arsenite and cyanide ion was confirmed by NMR, DFT optimization, ESI-MS, electrochemical behaviour and optical studies. This probe can also used for the determination of unknown concentrations of arsenite and cyanide ion in water using the calibration plot of known concentration vs. fluorescence intensity as well as in real water sample analysis.

Acknowledgements

NY is highly grateful to MHRD (Ministry of Human Resources and Development) to provide funding for this work.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra19781g

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