Development of real-time monitoring system for water based on chemiluminescence using chemiluminophore and photosensitizer for singlet oxygen generation

Abstract

This work provides a concept for developing real-time monitoring system for water based on chemiluminescence using a chemiluminophore and a photosensitizer for singlet oxygen (¹O₂) generation. Indeed, we demonstrated that the solution containing the two substances shows an increase in the chemiluminescence intensity with increasing the water content under photoirradiation.

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1. Introduction

The detection and quantification of water in samples and products, including solutions, solids, and gas, are absolutely crucial to environmental, biomedical and quality control monitoring systems, industrial process, food inspection and so on.^1–8 Therefore, several analytical methods and techniques for detecting and quantitating water in samples and products have been developed. As a common and classical technique for the quantification of water content, the Karl Fischer titration method, which utilizes volumetric or coulometric titration to determine the amount of water (0.001–100 wt%) in a solution or solid, is widely used in laboratory and industry. Indeed, the coulometric titration is suitable for the quantification of a low amount of water (0.001–1 wt%) which is based on the Karl Fischer reaction;

I₂ + SO₂ + 3Base + ROH + H₂O → 2Base·HI + Base·HSO₄R (1)

and

2I⁻ – 2e⁻ → I₂ (2)

1 mole of water reacts with 1 mole of iodine electrolytically generated by eqn (2), so that the electricity for 1 mg of water is equivalent to 10.71 coulombs. Thus, the Karl Fischer titration method offers sufficient quantitative accuracy of water content, but it is a batch (ex situ) analysis process, which leads to time-consuming measurements as well as the inability of real-time monitoring and flow (in situ) analysis of water.

In recent years, meanwhile, the optical sensing method utilizing organic colorimetric and fluorescent sensors for water is of considerable scientific and practical concern, because this method is a highly sensitive and quick flow analysis for the visualization as well as detection and quantification of water in samples and product.^9–26 Therefore, several investigations have been conducted on the design and synthesis of organic fluorescent sensors for water based on intramolecular charge transfer (ICT),^9–14 photo-induced electron transfer (PET),^15–19 Förster resonance energy transfer (FRET),^20,21 excited state intramolecular proton transfer (ESIP),^22–24 or solevatofluorochromism (SFC).^25–30 Indeed, it was revealed that these fluorescent sensors exhibit the changes in wavelength, intensity, and lifetime of fluorescence emission depending on the water content. Moreover, a definite advantage of fluorescent sensors over the Karl Fischer titration method is that it allows us to create functional dye materials, including the fluorescent sensor-polymer films and -immobilized substrates which have great potential for real-time monitoring and visualization of water on the surface of a substrate as well as water in solutions, solids, and gas.^1–5,17,18 On the other hand, if we can develop chemiluminescent sensors or chemi- and bioluminescence methods and techniques^31,32 for the detection and quantification of water, they are expected to be a high-contrast, fast-response, and real-time visualization system for water. It was found that 1,2-dioxetanones formed by reaction of 2-coumaranones with singlet oxygen (¹O₂), and 1,2-dioxetanes, which decomposes to produce chemiluminescence, and it is quenched by water (turn-off).^31–34 However, to our knowledge, there are no reports on the monitoring system for water based on enhancement of chemiluminescence (turn-on).

Thus, in this work, we designed and constructed the real-time monitoring system of water in solvents based on chemiluminescence using luminol as a chemiluminophore and phenazinone-based dye PZ2 as a photosensitizer for ¹O₂ generation (Fig. 1), because our previous work demonstrated that PZ2 exhibits the photoabsorption maximum at around 500 nm and possesses moderate ¹O₂ generation ability with ¹O₂ quantum yield (Φ_Δ) of 0.54 in THF.³⁵ In this system we assumed that the photoirradiation of the solution containing both luminol and PZ2 to generate the excited state of PZ2 results in the formation of 3-aminophthalate in the excited state by the reaction of luminol with ¹O₂ generated by the energy transfer from PZ2 in the triplet excited state to triplet oxygen (³O₂), gives rise to blue chemiluminescence. More interestingly, it was found that the solution showed the increase in the chemiluminescence intensity with an increase in the water content, so that it allows us to confirm the change in water content in the samples. Herein we report the proposed mechanism for the detection of water in solvent based on enhancement of chemiluminescence using a chemiluminophore and a photosensitizer for ¹O₂ generation and provide a concept for developing visualization and real-time monitoring system for water based on chemiluminescence.

Fig. 1 Proposed mechanism for detection of water based on chemiluminescence using luminol as a chemiluminophore and phenazinone-based dye PZ2 as a photosensitizer for ¹O₂ generation.

2. Results and discussion

In order to verify the chemiluminescence system based on luminol as a chemiluminophore and PZ2 as a photosensitizer for ¹O₂ generation, the photoabsorption and fluorescence spectral measurements of the basic aqueous solution which was prepared by using 5.0 × 10⁻² M NaOH aq, were performed, because for luminol the chemiluminescence originates from 3-aminophthalate in the excited state which is generated by the reaction of luminol with oxidizing agent under basic condition. Luminol (5.0 × 10⁻⁵ M) and PZ2 (5.0 × 10⁻⁵ M) in the basic aqueous solution showed the photoabsorption maximum (λ^abs_max) at 348 nm and 491 nm, respectively, with moderate molar extinction coefficient (ε_max = 7500 M⁻¹ cm⁻¹ for luminol and 7900 M⁻¹ cm⁻¹ for PZ2) (Fig. 2a). For the corresponding fluorescence spectra which are measured by using the excitation wavelength (λ^ex) at 348 nm for luminol and 509 nm for PZ2, the fluorescence maximum (λ^fl_max) appeared at 428 nm for lumimol and 616 nm for PZ2 (Fig. 2b). The fluorescence quantum yields (Φ_F) are ≤0.01 for luminol and ≤0.03 for PZ2. Meanwhile, when the fluorescence spectral measurement of the basic aqueous solution containing both luminol and PZ2 was performed by using the λ^ex at 509 nm, a luminescence band with a maximum wavelength at 445 nm as well as λ^fl_max at 590 nm for PZ2 were observed (Fig. 3a, see Fig. S1 for the photoabrorption spectrum and the fluorescence spectrum in the range of 550 nm to 700 nm). In addition, it was found that the luminescence spectrum of the basic aqueous solution containing both luminol and PZ2 by λ^ex at 509 nm coincide with the fluorescence spectrum of 3-aminophthalate in the basic aqueous solution (see Fig. S2). No fluorescence emission in the range of 350 to 500 nm was observed for luminol or PZ2 alone upon excitation at 509 nm, as luminol does not absorb in this wavelength and the fluorescence emission of PZ2 is at wavelengths higher than 500 nm. Furthermore, we performed the time dependent luminescence intensity measurement at 445 nm of the basic aqueous solution containing both luminol and PZ2 under photoirradiation at 509 nm. Indeed, when the solution was irradiated with excitation light at 509 nm after 10 seconds of monitoring of the luminescence intensity at 445 nm, the occurrence of the luminescence intensity was observed (Fig. 3b). It is worth noting here that the luminescence intensity increased rapidly with an increase in the photoirradiation time. Obviously, the fact strongly indicates that upon the photoirradiation with excitation light at 509 nm to the basic aqueous solution containing both luminol and PZ2, PZ2 in the ground state absorbs the light to generate the singlet excited state (¹PZ2*), and then the ¹PZ2* undergoes intersystem crossing (ISC) to form the triplet excited state (³PZ2*). Subsequent energy transfer from the ³PZ2* to ³O₂ gives rise to ¹O₂.³⁵ Consequently, the reaction of luminol with ¹O₂ results in the formation of 3-aminophthalate in the excited state that causes the blue chemiluminescence with a maximum wavelength (λ^cl_max) at 445 nm (Fig. 1).^36,37

Fig. 2 (a) Photoabsorption and (b) fluorescence spectra (λ^ex = 348 nm for luminol and λ^ex = 509 nm for PZ2) of luminol (5.0 × 10⁻⁵ M) and PZ2 (5.0 × 10⁻⁵ M) in 5.0 × 10⁻² M NaOH aqueous solution.

Fig. 3 (a) Luminescence spectra (in the range of 350 nm to 500 nm) of 5.0 × 10⁻² M NaOH aqueous solution containing only luminol (5.0 × 10⁻⁵ M), PZ2 (5.0 × 10⁻⁵ M), and both by (λ^ex) at 509 nm. (b) Time-dependent luminescence intensity at 445 nm of 5.0 × 10⁻² M NaOH aqueous solution containing both luminol and PZ2, where the solution was irradiated with excitation light at 509 nm after 10 seconds of monitoring of the luminescence intensity at 445 nm.

Therefore, the chemiluminescent detection ability of the system based on luminol and PZ2 for water in solvents was investigated by the luminescence spectral measurements (λ^ex = 509 nm) of the basic THF–water solution with various water content over a wide range 40 wt% to 100 wt% after photoirradiation with excitation light at 509 nm for ca. 3–10 min because NaOH is less soluble in the water content region below 40 wt%. It was found that the luminescence band with λ^fl_max at ca. 445 nm originating from 3-aminophthalate appeared and its intensity gradually increased with the increase in the water content from 40 wt% to 100 wt% (Fig. 4a). Indeed, the plot of the luminescence peak intensity at 445 nm versus the water fraction in the basic THF–water solution clearly demonstrated that the luminescence peak intensity increased almost linearly as a function of the water content in the range of 50–100 wt%, which is one of the factors required for the practical use of the luminescence detection system for water (Fig. 4b). Thus, this result demonstrates that the chemiluminescence system based on a chemiluminophore and a photosensitizer for ¹O₂ generation makes it possible to visualize, detect, and quantitate water in solvents.

Fig. 4 (a) Luminescence spectra (λ^ex = 509 nm) and (b) luminescence peak intensity at around 445 nm for the basic THF–water solution containing NaOH (5.0 × 10⁻² M), luminol (5.0 × 10⁻⁵ M) and PZ2 (5.0 × 10⁻⁵ M) with various water content (40–100 wt%) after photoirradiation with monochromatic light at 509 nm for ca. 3–10 min.

In order to investigate the influence of the ¹O₂ generation ability of PZ2 on the chemiluminescence intensity, we evaluated the ¹O₂ quantum yield (Φ_Δ) of PZ2 in the basic THF–water solution by using luminol, a ¹O₂ scavenger. The dissolved oxygen (DO) measurement at 25 °C for the basic THF–water solution showed that in the water content range of 40–100 wt% the DO concentration is almost constant (8.29–8.75 mg L⁻¹) irrespective of the water content (see SI for measurement of DO). Therefore, the ¹O₂ generation by PZ2 was investigated by monitoring the changes in photoabsorption spectra of luminol in the basic THF–water solution containing both PZ2 and luminol with various water content (40–100 wt%) upon photoirradiation with monochromatic light at 509 nm (300 μW cm⁻²). It was found that in the water content range of 40–100 wt%, the photoabsorption band of luminol at around 350 nm decreased with the increase in photoirradiation time, indicating that luminol reacted with ¹O₂ generated by the photosensitization of PZ2 (Fig. 5a for 80 wt%, see Fig. S3 for 40–100 wt%), resulting in 3-aminophthalate. To make clear the change in the ¹O₂ generation ability of PZ2 associated with the increase in the water content, the changes in optical density (ΔOD) at around 350 nm of luminol are plotted against the photoirradiation time for the water content range of 40–100 wt% (Fig. 5b), and the slope (m_sam) is used to estimate the Φ_Δ values: the Φ_Δ values for PZ2 were determined by the relative measurement method using the slope value (m_ref = −0.000318) of the plot for rose bengal (RB) (Φ_Δ = 0.75 in water)³⁸ as a reference photosensitizer (see SI for details). Indeed, the plots revealed that the correlation coefficient (R²) values of the calibration curves are 0.989–0.998 which indicate good linearity, and the m_sam values increases in the following order: −0.000215 (100 wt%) < −0.000245 (40 wt%) < −0.000279 (50 wt%) < −0.000288 (60 wt%) < −0.000374 (90 wt%) < −0.000389 (70 wt%) < −0.000398 (80 wt%). Therefore, as with case of the m_sam values, the Φ_Δ values also increases in the following order: 0.49 (100 wt%) < 0.56 (40 wt%) < 0.62 (50 wt%) < 0.65 (60 wt%) < 0.88 (70 wt%) < 0.90 (90 wt%) < 0.95 (80 wt%). Thus, this result shows that the Φ_Δ values increased with increase in the water content in the range of 40–90 wt%, and then considerably decreed in the water content 100 wt%, that is, only water. The difference in Φ_Δ values against water content will be discussed later.

Fig. 5 (a) Photoabsorption spectra for the photooxidation of luminol (1.0 × 10⁻⁴ M) in the presence of PZ2 (abs. @509 nm = ca. 0.03) in the basic THF–water solution containing NaOH (5.0 × 10⁻² M) with water content of 80 wt% under photoirradiation with 509 nm (300 μW cm⁻²). Inset is magnifications of maxima in the spectra at around 350 nm. (b) Plots of ΔOD at around 350 nm for the photooxidation of luminol in the presence of PZ2 (abs. @509 nm = ca. 0.03) in the basic THF–water solution containing NaOH (5.0 × 10⁻² M) with various water content (40–100 wt%) against the photoirradiation time (509 nm, 300 μW cm⁻²).

Thus, in order to confirm the factors contributing to the enhancement of chemiluminescence intensity with the increase in water content in basic THF–water solution containing luminol and PZ2, we estimated the chemiluminescence quantum yield (Φ_CL), chemical yield (Φ_R), Φ_F and chemiexcitation efficiency (Φ_S); the Φ_CL is a product of three efficiencies, that is, Φ_CL = Φ_R × Φ_F × Φ_S; the Φ_R of 3-aminophtalate by chemiluminescent reaction, the Φ_F of 3-aminophtalate and the Φ_S for generating the singlet-excited state of 3-aminophtalate.^39,40 The Φ_CL values were estimated as values relative to the Φ_CL value (0.012)⁴¹ of luminol in water which are based on the chemiluminescence intensity, the Φ_R values were evaluated as an equivalent to the Φ_Δ values, which are based on the changes in optical density (ΔOD) of luminol by the reaction with ¹O₂, the Φ_F values were estimated by using 3-aminophthalic acid in the basic THF–water solution containing NaOH, and thus the Φ_S values were calculated by using Φ_CL = Φ_R × Φ_F × Φ_S. (Table 1). However, the Φ_CL values based on the chemiluminescence intensity were estimated assuming that the reaction kinetics of luminol with ¹O₂ does not change with the water content, and thus might not be exact. The plots of Φ_CL, Φ_R, Φ_F, and Φ_S values against water content revealed that the Φ_R and Φ_S values gradually increased with increase in the water content in the range of 40–90 wt%, while the Φ_R and Φ_S values significantly decreased and increased, respectively, when the water content reached 100 wt% (Fig. 6). On the other hand, the Φ_F values are almost constant (0.30–0.34) in the water content range of 40–100 wt%. Therefore, this result indicated that in the water content range of 40–90 wt%, the increase in Φ_CL value with increasing water content is dependent on the increase in Φ_R and Φ_S values. One can see that low Φ_R values are the main factor leading to low Φ_CL values, especially in the low water content region. Interestingly, it was found that the highest Φ_CL value was obtained in the water content 100 wt% that is due to the highest Φ_S value, but it was the lowest Φ_R value. Consequently, this work revealed that for the chemiluminescence system based on luminol and a photosensitizer PZ2 for ¹O₂ generation, the increase in chemiluminescence intensity, that is, Φ_CL value, with increasing water content is attributed to increase in the Φ_S value as well as the Φ_R (Φ_Δ) value.

Table 1 Chemiluminescent data of luminol with ¹O₂ generated by PZ2 in basic THF–water solution containing various water content

Water content/wt%	Φ R	Φ F	Φ S	Φ CL
a Chemical yield of 3-aminophtalate by chemiluminescent reaction. b Fluorescence quantum yield of 3-aminophtalate. c Chemiexcitation efficiency for generating the singlet-excited state of 3-aminophtalate. d Chemiluminescence quantum yield.
40	0.56	0.34	0.014	0.003
50	0.62	0.33	0.016	0.003
60	0.65	0.33	0.019	0.004
70	0.88	0.34	0.019	0.006
80	0.95	0.30	0.029	0.008
90	0.90	0.30	0.039	0.011
100	0.49	0.29	0.083	0.012

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Fig. 6 The plots of (a) Φ_R, Φ_F, (b) Φ_CL, and Φ_S values against various water content (40–100 wt%) in basic THF–water solution containing NaOH (5.0 × 10⁻² M), luminol (5.0 × 10⁻⁵ M) and PZ2 (5.0 × 10⁻⁵ M).

3. Conclusions

In this work, we have designed and constructed the chemiluminescence system based on luminol as a chemiluminophore and phenazinone-based dye PZ2 as a photosensitizer for ¹O₂ generation, which would make it possible to visually confirm the presence of water in the samples: the formation of 3-aminophthalate in the excited state by the reaction of luminol with ¹O₂ which is generated by the energy transfer from PZ2 in the triplet excited state to ³O₂ gives rise to the blue chemiluminescence. In fact, the system shows an enhancement of chemiluminescence intensity with an increase in the water content in the solution. Furthermore, we revealed the mechanism for detecting water based on the fact that not only the ¹O₂ generation ability of PZ2 but also chemiexcitation efficiency for generating the singlet-excited state of 3-aminophtalate are enhanced with increasing the water content in the basic THF–water solution. Consequently, this work provides a concept for developing visualization and real-time monitoring system for water based on enhancement of chemiluminescence using a chemiluminophore and a photosensitizer for ¹O₂ generation. Further studies on development of strongly basic group-substituted photosensitizing chemiluminescent phenazinone-based dyes possessing both chemiluminescence and ¹O₂ generation abilities, which produce chemiluminescence with a trace amount of water under natural light or interior light, are now in progress.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d5nj03547c.

Acknowledgements

This work was supported by Grants-in-Aid for Scientific Research (B) from the Japan Society for the Promotion of Science (JSPS) KAKENHI Grant Number 22H02123 and by Fuso Innovative Technology Fund.

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