Development of highly-sensitive fluorescence PET (photo-induced electron transfer) sensor for water: anthracene–boronic acid ester

Abstract

Anthracene–boronic acid ester OF-2 having a cyano group as an electron-withdrawing substituent was designed and developed as a highly-sensitive fluorescence PET sensor for detection of a trace amount of water in various solvents (polar, less polar, protic and aprotic solvents).

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Organic fluorescent sensors for detection and quantification of a trace amount of water in solids, solutions, the atmosphere and products have created considerable interest in recent years from the viewpoint of not only fundamental study in analytical chemistry, photochemistry and photophysics, but also their potential applications in environmental and quality control monitoring systems in industry and sanitary and medical materials.^1–11 In most of fluorescent sensors developed so far for a trace amount of water, which is based on fluorescent conjugated polymer and organic fluorescent dyes, the fluorescence intensity decreases as the amount of water increases in organic solvents and this feature can be attributed to the aggregation of sensors or the formation of hydrogen bonding between the sensor and water molecules with the increase in the water content.^1–7 However, this fluorescence quenching system makes it difficult to detect trace amounts of water. Additionally, the detection and quantification of water based on the fluorescence quenching system depend strongly on the kind of solvents (polar, less polar, protic and aprotic solvents). On the other hand, recently, as a new approach for improving the detection limit of water, we have devised the fluorescence enhancement system based on PET (photo-induced electron transfer),¹² that is, anthracene–boronic acid ester OM-1 was designed and synthesized as a fluorescence PET sensor for water (Scheme 1).¹³ An anthracene–boronic acid system was developed by Shinkai group¹⁴ and Wang group¹⁵ as a fluorescence PET sensor for saccharides. In OM-1, boronic acid ester enhances the solubility of the sensor in organic solvents. The addition of water to organic solvents containing OM-1 causes efficient formation of fluorescent ionic structure OM-1a, resulting in the suppression of PET (occurrence of fluorescence) due to protonation of the tertiary amino group. The detection limit (DL) and quantitation limit (QL) were as low as 0.04 and 0.1 wt%, respectively, for both acetonitrile and ethanol (Table 1). However, the DL and QL in less polar organic solvents such as 1,4-dioxane and THF were higher than those in polar organic solvents (acetonitrile and ethanol).

Scheme 1 Proposed mechanisms of fluorescence PET sensors (a) OM-1, OF-1 and OF-2 for detection of water in organic solvents.

Table 1 DL and QL of OM-1, OF-1 and OF-2, and of previously reported fluorescence water sensors for water determination in various organic solvents

Sensor	Solvent	ms	DL	QL
OM-1 (ref. 12)	1,4-Dioxane	14	0.2 wt%	0.7 wt%
	THF	19	0.2 wt%	0.5 wt%
	Acetonitrile	67	0.04 wt%	0.1 wt%
	Ethanol	106	0.04 wt%	0.1 wt%
OF-1	1,4-Dioxane	12	0.3 wt%	0.8 wt%
	THF	6.7	0.5 wt%	1.5 wt%
	Acetonitrile	55	0.06 wt%	0.2 wt%
	Ethanol	86	0.04 wt%	0.1 wt%
OF-2	1,4-Dioxane	334	0.01 wt%	0.03 wt%
	THF	390	0.008 wt%	0.026 wt%
	Acetonitrile	382	0.009 wt%	0.026 wt%
	Ethanol	362	0.009 wt%	0.027 wt%
Ref. 3	1,4-Dioxane	−40.23	0.008 wt%	No data
	Acetonitrile	−52.10	0.006 wt%	No data
	Ethanol	−22.42	0.015 wt%	No data
Ref. 6	DMF	−722.3	0.008 wt%	0.03 wt%
	NMP	−610.2	0.009 wt%	0.03 wt%
	Acetonitrile	−275.4	0.02 wt%	0.07 wt%
	Ethanol	−75.6	0.1 wt%	0.3 wt%

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Thus, in this work, to provide a direction in molecular design toward creating highly-sensitive fluorescence PET sensor for a trace amount of water in various solvents, we have designed and synthesized anthracene–boronic acid ester OF-1 and OF-2 having methoxy group as an electron-donating substituent and a cyano group as an electron-withdrawing substituent, respectively, at the para position on benzeneboronic acid ester (Scheme 1; see ESI† for synthetic procedures). This work indicates that the introduction of electron-withdrawing substituent on the benzeneboronic acid ester can enhance the Lewis acidity of the boron atom, effectively leading to formation of fluorescent ionic structure by addition of water molecules.

Absorption and fluorescence spectra of OF-1 and OF-2 were measured in 1,4-dioxane, THF, acetonitrile and ethanol that contained various concentrations of water. As shown typically in Fig. 1 for acetonitrile (see ESI† for 1,4-dioxane, THF and ethanol), the absorption spectra of OF-1 and OF-2 in all the four solvents did not undergo appreciable changes in intensity and shape upon addition of water. In contrast, the corresponding fluorescence spectra of OF-1 and OF-2 exhibited significant changes in intensity with a negligible change in their spectral shapes. The changes in the fluorescence peak intensity are plotted in Fig. 2 against the water fraction of these four organic solvents. For OF-1, in the low water content region below 1.0 wt%, the fluorescence intensities increased almost linearly with the increase in water content for all four solvents. However, the slopes for 1,4-dioxane and THF are smaller than those for acetonitrile and ethanol (Fig. 2c). When the water content ranges between 1 and 5 wt%, the fluorescence intensity increased gradually for acetonitrile and ethanol (Fig. 2a), while for 1,4-dioxane and THF the fluorescence intensity increased dramatically. The features of fluorescence enhancement for OF-1 with the increase in the water content are similar to those for OM-1. On the other hand, for OF-2, in the low water content region below 1.0 wt%, the fluorescence intensities increased dramatically and almost linearly with the increase in the water content for all four solvents (Fig. 2d), although the linear region of the plot for ethanol is narrow (0.016–0.20 wt%). The fluorescence intensities level off in the water content region greater than 1.0 wt% for all four solvents (Fig. 2b). It is particularly worth noting that for both OF-1 and OF-2 the plots for 1,4-dioxane, THF and acetonitrile fit straight lines passing through the origin, which are required for the practical use of the fluorescence sensor for water. These results indicate that the fluorescence enhancement of OF-1 and OF-2 with the increase in the water content can be attributed to suppression of PET due to the formation of OF-1a and OF-2a with a stable fluorescent ionic structure between the protonated tertiary amino group and the hydroxylated boronic acid ester (Scheme 1). Consequently, the enhanced fluorescence of OF-1 and OF-2 in anhydrous ethanol may be attributed to the suppression of PET by the hydrogen bonding between hydroxyl group of ethanol and the amino group of OF-1 and OF-2.

Fig. 1 (a) Absorption and (b) fluorescence spectra (λ_ex = 366 nm) of OF-1 (c = 2.0 × 10⁻⁵ M) in acetonitrile containing water (0.014–10 wt%). (c) Absorption and (d) fluorescence spectra (λ_ex = 366 nm) of OF-2 (c = 2.0 × 10⁻⁵ M) in acetonitrile containing water (0.034–10 wt%).

Fig. 2 Fluorescence peak intensity of (a) OF-1 and (b) OF-2 at around 413 nm (λ_ex = 366 nm) as a function of water content in 1,4-dioxane, THF, acetonitrile and ethanol in a water-content region below 10 wt%, and the fluorescence peak intensity of (c) OF-1 and (d) OF-2 in 1,4-dioxane, THF, acetonitrile and ethanol in a low water-content region below 1.0 wt%.

To gain insight into the detection and quantification of water in organic solvents, the calibration equations for the determination of water in organic solvents were obtained from Fig. 2c and d and the example of OF-2 is as follows (see ESI† for OF-1):

1,4-Dioxane: F = 334.0[H₂O] + 4.0 (R² = 0.962, [H₂O] = 0.014–0.61 wt%) (1)

THF: F = 390.3[H₂O] + 5.7 (R² = 0.978, [H₂O] = 0.026–0.40 wt%) (2)

Acetonitrile: F = 382.5[H₂O] + 2.4 (R² = 0.986, [H₂O] = 0.034–0.40 wt%) (3)

Ethanol: F = 362.4[H₂O] + 82.3 (R² = 0.977, [H₂O] = 0.016–0.20 wt%) (4)

The slope (m_s) values for OM-1, OF-1 and OF-2 are summarized in Table 1. The m_s value for OF-1 becomes steeper in the following order: THF (m_s = 6.7) < 1,4-dioxane (m_s = 12) < acetonitrile (m_s = 55) < ethanol (m_s = 86), that is, the m_s values in less polar organic solvents (1,4-dioxane and THF) are much smaller than those in polar organic solvents (acetonitrile and ethanol), as with the case of OM-1. However, the m_s values for OF-1 in all four solvents are smaller than those of OM-1. On the other hand, it is worth mentioning that for OF-2 there is a little difference in the m_s value among the four solvents, and the m_s values (ca. 330–390) for OF-2 in all four solvents are much larger than those of OM-1 and OF-1. The large m_s values for the cyano-substituted sensor OF-2 relative to the unsubstituted sensor OM-1 can be attributed to the fact that for OF-2 the cyano group at the para position on benzeneboronic acid ester enhances the Lewis acidity of the boron atom due to its electron-withdrawing substituent, leading to the facilitation of the formation of fluorescent ionic structure by addition of water molecules. In contrast, for OF-1 the electron-donating methoxy group at the para position on benzeneboronic acid ester diminishes the Lewis acidity of the boron atom, leading to the retardation of the formation of fluorescent ionic structure by addition of water molecules.

We estimated the DL and QL based on the following equations: DL = 3.3σ/m_s and QL = 10σ/m_s, where σ is the standard deviation of blank sample and m_s is the slope of calibration curve in the region with a low water content below 1.0 wt%, respectively (Table 1). The DL and QL of OF-1 are, respectively, 0.3 and 0.8 wt% for 1,4-dioxane and 0.06 and 0.2 wt% for acetonitrile, which are inferior to those of OM-1. Thus, for the both OM-1 and OF-1 the DL and QL in polar organic solvents (acetonitrile and ethanol) were lower than those in less polar organic solvents (1,4-dioxane and THF). On the other hand, the DL and QL of OF-2 are, respectively, 0.01 and 0.03 wt% for 1,4-dioxane and 0.009 and 0.026 wt% for acetonitrile, which are superior to those of OM-1 and OF-1, and are equivalent to or superior to those of the reported fluorescence water sensors based on a fluorescence quenching system (ref. 3 and 6 in Table 1) by the aggregation of sensors or the formation of hydrogen bonding between sensor and water molecules with the increase in the water content. Consequently, it was found that anthracene–boronic acid ester OF-2 having a cyano group as an electron-withdrawing substituent acts as a highly sensitive fluorescence PET sensor for the detection of a trace amount of water in polar, less polar, protic and aprotic solvents. This implies that the introduction of electron-withdrawing substituent on the benzeneboronic acid ester can enhance the Lewis acidity of the boron atom, effectively leading to formation of fluorescent ionic structure by addition of water molecules.

In conclusion, to gain insight into the substituent effect on the sensing ability of fluorescence PET sensor for detection of a trace amount of water, we have designed and synthesized anthracene–boronic acid ester OF-1 and OF-2 having a methoxy group as an electron-donating substituent and a cyano group as an electron-withdrawing substituent, respectively, at the para position on benzeneboronic acid ester. It was found that the DL and QL of the cyano-substituted sensor OF-2 in various solvents are much lower than those of the methoxy-substituted sensor OF-1 and the unsubstituted sensor OM-1. Thus, we have demonstrated that a key point for creating a highly-sensitive fluorescence PET sensor for a trace amount of water based on anthracene–boronic acid ester is to enhance the Lewis acidity of boronic acid ester, leading to the facilitation of the formation of fluorescent ionic structure by addition of water molecules. However, the detection limit (DL) of water by Karl Fischer titration method is a few ppm (<0.001 wt%). Thus, to improve further the PET method is necessary, much effort to develop highly-sensitive fluorescence PET sensor for water is necessary.

Acknowledgements

This work was supported by Konica Minolta Science and Technology Foundation for Konica Minolta Imaging Science Encouragement Award.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Details of experimental procedures, synthesis and characterization of compound. See DOI: 10.1039/c4ra02265c

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