Selective detection of nitrite ion by an AIE-active tetraphenylethene dye through a reduction step in aqueous media

Abstract

This paper reports a highly selective nitrite ion receptor based on an AIE-active tetraphenylethene-bearing amino functionality. In water, the receptor has been shown to be selective for nitrite ion (NO₂⁻) over other anions (NO₂⁻, CH₃COO⁻, NO₃⁻, CO₃²⁻, S₂O₃²⁻, SO₄²⁻, HSO₃⁻, and Cl⁻), with pronounced changes in absorption characteristics i.e. red-shift of ∼30 nm. The visual observation of colour change, shift in absorption, and enhancement of emission, clearly show that this receptor is very selective and sensitive to nitrite ions.

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In recent years, air/water pollution has become one of the major problems in urban areas, with the combustion of fossil fuels used in power plants, vehicles, and other incineration processes, being the main source of pollutants.¹ The main combustion-generated air contaminants are nitrite (NO₂⁻) ions, which are considered primary pollutants of the atmosphere as they are responsible for photochemical smog, acid rain, and ozone layer depletion, and are thus a great threat to human health.² Nitrite ions, upon interaction with proteins, act as important precursors for the generation of highly carcinogenic N-nitrosamines,³ and their excessive consumption can lead to a number of medical issues such as oesophageal cancer, spontaneous abortion, infant methemoglobinemia, birth defects in the central nervous system, and so on.⁴ The U.S. Environmental Protection Agency (EPA) has legislated a maximum contaminant level of nitrite to 1 ppm (21.7 μM), and the World Health Organisation (WHO) has set 3 ppm as a guideline.⁵ Because of the high reactivity and adverse effects of these species, rapid detection strategies are of extreme importance in monitoring drinking water quality, and in clinical diagnosis.

The most common techniques used for the detection of nitric oxide, include electrochemical, gas chromatographic, fluorescence, and colorimetric methods.⁶ Literature search has revealed several reports on the detection of nitrite ion, such as chemical and optical, ion chromatographic, and electrochemical methods, and also the commercially-available Griess reagent kit.^7–12 Of these reported techniques, colorimetric methods are the most simple and convenient for on-site visual analysis of these biologically-important analytes. However, even though these methods offer certain benefits, they also have limitations, such as poor specificity and the use of expensive experimental apparatus, which restrict their application in practice. Therefore, a simple, sensitive and sensitive sensor for the detection of nitrite ions could greatly aid individuals in evaluating the quality of drinking water and in clinical diagnosis.

Recently, the use of mechano-luminescent molecules, typically, small organic molecules whose emissions are very weak in dilute solution, but become highly luminescent in the aggregated- and solid-film-state (i.e. demonstrate Aggregation-Induced Emission (AIE)), have stimulated significant research interest.¹³ Taking advantage of this behaviour, various groups have used AIE-active derivatives for multiple applications, especially in cell imaging, optical devices, electroluminescent materials, and chemo/biosensors, etc.¹⁴ Amongst these AIE-active molecules, containing the tetraphenylethene (TPE) chromophore have been widely used, due to their ease of synthesis and simple functionalization strategies.^14h–k

Herein, we report the development of a simple and novel probe based on an amino-functionalised, AIE-active TPE chromophore i.e. tetraamino-tetraphenylethene 1 (TA-TPE), which acts as a sensitive probe for nitrite ions, and signals the event through a visible colour change in aqueous media (Fig. 1). The synthesis of TA-TPE (1) has been described in our earlier report.¹⁵ The selectivity for NO₂⁻ was determined in the presence of other anions including: SO₄²⁻, Cl⁻, HSO₃⁻, CO₃²⁻, CH₃COO⁻, NO₃⁻, and S₂O₃²⁻ through naked-eye detection (Fig. 1b). Our probe, TA-TPE (1), offers three additional benefits over previously reported TPE-based receptor systems:¹⁶ the sensitive detection of NO₂⁻ through a reduction step; “turn-on” fluorescence rather than “turn-off”; and the colorimetric response of receptor 1 (Fig. 1b), which clearly shows discriminate selectivity for nitrite over other competitive anions. Based on our findings, we propose a possible mechanism for the detection of nitrite ion, through diazotization of the protonated form of probe 2, and the nitrite ions present in water, to yield the diazonium salt, 3. The salt 3 undergoes in situ hydrolysis in aqueous media to give the hydroxyl-substituted product 4 (4OH-TPE), as shown in Fig. 1a.^11,12

Fig. 1 (a) A possible mechanism for the detection of nitrite ions using TA-TPE (1), and (b) colorimetric response of TA-TPE (1) toward various competitive anions.

The UV-vis absorption spectrum of 1 in methanol (MeOH) gives two typical peaks at 273 nm and 345 nm. However, upon protonation with HCl (0.1 N), the first absorption peak shifts to 301 nm, and finally, on addition of NaNO₂, TA-TPE (1) converts into 4OH-TPE (4) and the absorption peak at 331 nm shifts by a furthers 30 nm (Fig. 2a). Furthermore, UV-vis absorption spectroscopy was used to investigate the selective sensing ability of receptor TA-TPE (1; 15 μM) on addition of various anions (50 μM), as shown in Fig. 2b. The UV-vis absorption peak of receptor TA-TPE (1) appeared at 301 nm, but on addition of CH₃COO⁻, NO₃⁻, CO₃²⁻, S₂O₃²⁻, SO₄²⁻, HSO₃⁻, and Cl⁻ (as their sodium salts) to receptor TA-TPE (1) no specific changes in the absorption spectra were observed. The ∼30 nm absorption shift was only observed in the presence of NO₂⁻ (Fig. 2b).

Fig. 2 Normalized absorption spectra of TA-TPE (1) (15 μM) in (a) respective solutions (λ_ex = 300 nm), and (b) in the presence of 50 μM of the anions such as NO₂⁻, CH₃COO⁻, NO₃⁻, CO₃²⁻, S₂O₃²⁻, SO₄²⁻, HSO₃⁻, and Cl⁻ (as their sodium salt), which demonstrate the selective detection of NO₂⁻.

To understand the sensing capability of receptor 1 with NaNO₂ in depth, a systematic, concentration-dependent titration of NaNO₂ with TA-TPE (1) was performed, as shown in Fig. 3. The UV-vis absorption spectra clearly show that upon incremental addition of NO₂⁻ (as its sodium salt) to receptor TA-TPE (1) a gradual decrease in the intensity of peak 301 nm results, along with increasing intensity of the 331 nm peak, and the isosbestic point at 316 nm (Fig. 3a). Fluorescence emission spectroscopy was also employed to investigate the sensing ability of TA-TPE (1). As expected, TA-TPE (1) upon protonation i.e. 2, gave a very weak emission at 510 nm (λ_ex = 300 nm). Interestingly, an enhancement of the fluorescence intensity, of about 20 fold, was observed on incremental addition of nitrite ion (0–50 equiv.), as shown in Fig. 3b, indicating that the probe could detect trace levels of nitrite ions in solution, efficiently. We believe that compound 1, in protonated form, appears to be in the solution-state, however, 4OH-TPE (4) (the aggregated form) is not completely soluble in water, and thus, enhancement of emission is observed. Since the TPE-dyes are non-fluorescent in solution and highly fluorescent in the aggregated- and solid-state, so called ‘AIE-active’.^13–15

Fig. 3 (a) and (b) Changes in absorption and fluorescence response of probe 1 (15 μM, protonated using 0.1 N HCl) upon addition of different concentrations of nitrite (0–50 μM) in water, respectively. Inset in “b”: plot of relative emission intensity of 1 against the concentration range of nitrite ion, showing excellent fit (R² = 0.0996).

Fig. 4 shows a selectivity plot indicating the changes in absorbance (331 nm) of receptor 1 (15 μM, protonated form) on addition of various competitive anions, such as NO₂⁻, CH₃COO⁻, NO₃⁻, CO₃²⁻, S₂O₃²⁻, SO₄²⁻, HSO₃⁻, and Cl⁻ under similar conditions. Negligible change was observed for anions other than nitrite, even upon addition of 100 fold greater concentrations of these ions. TA-TPE (1) responds to NO₂⁻ ions below micromolar concentration ranges, and has an estimated detection limit of 17.7 ppb of NaNO₂ (Fig. 4b), sensitivity of TA-TPE towards nitrite ions is higher than that of recently reported for aza-BODIPY dye i.e. 20 ppb.^11,12 Our sensing system is able to detect nitrite ions at >1 ppm, which is the maximum contaminant level, set by the EPA for drinking water.⁵ ESI mass spectroscopy confirmed our hypothetical mechanism (outlined in Fig. 1) for nitrite-sensing through a reduction process, and gives an m/z peak at 396.1369, which matches perfectly with the calculated value for compound 4 (4OH-TPE) i.e. 396.1362.

Fig. 4 (a) Selectivity plot showing the changes in maximum absorbance response of probe 1 (15 μM, protonated 0.1 N aqueous HCl) at 331 nm by addition of various competitive anions under identical conditions. (b) A calibration plot for successive addition of nitrite against 1 in 0.1 N aqueous HCl, showing an excellent linear fit with R² = 0.9863.

Fig. 4 shows a selectivity plot indicating the changes in absorbance (331 nm) of receptor 1 (15 μM, protonated form) on addition of various competitive anions (50 μM), such as NO₂⁻, CH₃COO⁻, NO₃⁻, CO₃²⁻, S₂O₃²⁻, SO₄²⁻, HSO₃⁻, and Cl⁻ under similar conditions. Negligible change was observed for anions other than nitrite, even upon addition of 100 fold greater concentrations of these ions. TA-TPE (1) responds to NO₂⁻ ions below micromolar concentration ranges, and has an estimated detection limit of 17.7 ppb of NaNO₂ (Fig. 4b), sensitivity of TA-TPE towards nitrite ions is higher than that of recently reported for aza-BODIPY dye i.e. 20 ppb level.^11,12 Our sensing system is able to detect nitrite ions at >1 ppm, which is the maximum contaminant level, set by the EPA for drinking water.⁵ ESI mass spectroscopy confirmed our hypothetical mechanism (outlined in Fig. 1) for nitrite-sensing through a reduction process, and gives an m/z peak at 396.1369, which matches perfectly with the calculated value for compound 4 (4OH-TPE) i.e. 396.1362. Fig. 5 shows time dependent ¹H NMR (D₂O) spectra of 4NH₂-TPE to 4OH-TPE in the presence of NO₂⁻ ions (10 equiv.), it clearly shows quick response with addition of nitrite ion.

Fig. 5 ¹H NMR of TA-TPE (10 μM) in the presence of NO₂⁻ (10 equiv.) ion: (a) a protonated TA-TPE salt 2, (b) in situ formation of intermediate 3 within 30 second and (c) fully converted to 4OH-TPE within 2 min.

In conclusion, we have developed a novel AIE-active colorimetric TPE-based receptor for the selective detection of nitrite ion in water. The receptor shows visual colour changes from colourless to dark yellow, with a detection limit of 17.7 ppb level. These results reveal that TA-TPE (1) has excellent selectivity toward nitrite ions (NO₂⁻), even in the presence of other competitive anions such as CH₃COO⁻, NO₃⁻, CO₃²⁻, S₂O₃²⁻, SO₄²⁻, HSO₃⁻, and Cl⁻ and thus can be used for the efficient detection of nitrite ions in samples of different origin. The drastic colour change and strong fluorescence change of TA-TPE (1) are simply driven by reduction of amine to hydroxyl group. The response time was only a few seconds in water, and we were able to easily follow the response by absorption, naked eye or emission changes. This method offers additional benefits in that the receptor can be employed for the sensitive detection of NO₂⁻, through a reduction step, along with a shift in absorption and “turn-on” fluorescence. This method is selective and environmentally viable.

Acknowledgements

S. V. B. acknowledges financial support from the Australian Research Council (ARC), Australia, under a Future Fellowship Scheme (FT110100152).

Notes and references

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