A Water Indicator Strip: Instantaneous Fluorogenic Detection of Water

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A Water Indicator Strip: Instantaneous Fluorogenic Detection of Water in Organic Solvents, Drugs, and Foodstuffs Tae-Il Kim, and Youngmi Kim Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b00270 • Publication Date (Web): 22 Feb 2017 Downloaded from http://pubs.acs.org on February 22, 2017

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A Water Indicator Strip: Instantaneous Fluorogenic Detection of Water in Organic Solvents, Drugs, and Foodstuffs Tae-Il Kim and Youngmi Kim* Department of Chemistry, Kyung Hee University, 126 Kyungheedae-ro, Dongdaemun-gu, Seoul, 02447, Korea ABSTRACT: A simple, highly sensitive and rapid colorimetric and fluorescent indicator 1 has been developed for the qualitative and quantitative determination of water. The water-induced sensitive (LOD = 0.003%, v/v) and fast (< 10 s) change in emission properties was applied to the determination of water in organic solvents, drugs, and foodstuffs in both solution and practical solid state indicator paper strips.

Water contamination can adversely affect many organic reactions. Moreover, control and analysis of moisture levels are critical in products including pharmaceuticals and foodstuffs.1 Small amounts of water can also induce corrosion and substantial damage on production lines or equipment. In particular, jet fuel must be free of water contamination for the safe operation of aircrafts.2 Accordingly, efficient methods for the timely detection of amounts of water are highly valuable not only in synthetic laboratories but also for industrial applications. The Karl Fischer (K-F) titration is among the most commonly used methods for the determination of water content through coulometric (1-5 ppm to 5%) and volumetric (100 ppm up to 100%) analysis.3,4 The selective reaction between the K-F reagent (SO2/I2/base) and water provides the K-F method high selectivity and accuracy. Nevertheless, it is limited by the need for specialized lab equipment and relatively lengthy assays. In addition, undesirable side reactions of alcoholic K-F solutions make a number of substances, such as redox-active compounds, incompatible with these assays.5 Fluorescence-based systems for water detection have emerged as alternatives to the K-F titration, owing to their operational simplicity, high sensitivity, and low cost.6 For most fluorescent probes, the mechanisms for fluorescence “on-off” or “off-on” switching rely on a water-induced proton transfer or non-covalent modulation of photophysical processes including photoinduced electron transfer (PeT),7 intramolecular charge transfer (ICT),8 excited-state intramolecular proton transfer (ESIPT),9 or π-stacking aggregation.10 Despite notable successes, the output signal of these probes is highly sensitive to environmental factors such as polarity11 or pH of the sensing media.12 Rare examples of water-specific chemical probes have been introduced to provide more robust detection schemes,13,14 focusing on the hydrolytic cleavage of Schiff bases.13 In this work, we report probe 1 as a simple, rapid, and sensitive colorimetric and fluorescent indicator for the determination of water. Assays were developed for water in organic solvents, drugs, and foodstuffs, in both solution and practical solid state indicator strips.

Scheme 1. (A) Structures of 3,5-dimethyl BODIPYs substituted at the meso-position with an electron-withdrawing group (EWG) or electron-donating groups (EDG). (B) Proposed fluorescence sensing scheme for water. (A) 6

EDG

EWG 8

7

1 2

N N 3 B F 4 F red-emissive

vs.

5

EWG: CHO (1) (B)

O

N N B F F green-emissive

EDG: CH3 (2), CH(CH3)2 (3), CH2OH (4) HO

C H

OH C H

H2O N N B F F 1

N N B F F 1•(H2O)

Our approach exploits the reversible hydration of aldehydes to give aldehyde hydrates (1,1-diols), which is coupled with the unique photophysical properties of meso (C8)-substituted boron dipyrromethene (BODIPY) dyes to result in a fluorogenic sensing response. We previously showed that the absorption and emission profiles, and the fluorescence quantum yields of meso-substituted BODIPY dyes are highly dependent on the electronic properties of the mesosubstituents.15 The introduction of electron-withdrawing mesosubstituents leads to bathochromic shifts with respect to corresponding analogues with alkyl substituents at the mesoposition. The former preferentially stabilize the LUMO level and consequently reduce the HOMO-LUMO gap. Inspired by these, we studied the meso-formyl-3,5-dimethyl BODIPY (1, Scheme 1), as a colorimetric and fluorescent indicator for water. Probe 1, in which conjugation of the formyl substituent with dipyrrin is optimal (dihedral angle is almost 0°),15b shows a bright red fluorescence around at 620 nm in solution. By comparison, the corresponding meso-alkyl-substitutedBODIPYs (2: meso-methyl; 3: meso-isopropyl; 4: meso-

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hydroxymethyl) are intensely green emissive around at 525 nm.15b,16 We hypothesized that upon contact with water, the red emissive probe 1 would convert to the aldehyde hydrate (1,1-diol) 1⋅⋅(H2O) (Scheme 1B), resulting in significant absorption and emission shifts as the meso-substituent goes from a withdrawing to a donating group and the extent of πconjugation is reduced.

EXPERIMENTAL SECTION Assay Studies for Water Contents in Organic Solvents. The solution of probe 1 (1 mM, 2 µL) dissolved in each dry organic solvent was added to each organic solvent−water mixture solution (198 µL) containing various concentrations of water (0−99%, v/v). All spectra were obtained immediately after the addition of probe 1 to each solution at 25 °C. Paper Strip-based Detection of Water in Organic Solvents. To facilitate the use of probe 1 for the detection of water in a portable manner, a simple paper strip (10 mm x 10 mm) loaded with probe 1 was prepared by dipping filter paper into a THF solution of probe 1 (1 mM) for 10 seconds and drying it under reduced pressure. For the detection of water, the dynamic changes in emission color of as-prepared paper strip were observed upon dropping of THF–water mixture soultion (5 µL) onto the paper strip or immersing the strip into 1 mL of THF–water mixture soultion at 25 °C under 365 nm UV light illumination. Determination of Water Contents in Various Samples. The solid-powdered active pharmaceutical ingredients (A: Atorvastatin calcium trihydrate, T; Tenofovir disoproxil fumarate) were obtained from Sungwun Pharmacopia/Bio Inc. 10 mg of each sample was dissolved in dry THF (1 mL), and sonicated for 15 min to dissolve completely. Each sample solution (198 µL) was mixed with the probe 1 dissolved in dry THF (1 mM, 2 µL) in 96-well plate set-up. Final concentration of probe 1 in solution was 10 µM. Fluorescence spectra were obtained immediately after mixing of probe 1 with each sample, and fluorescence intensity at 526 nm was recorded. Using calibration curve obtained from Figure 5B, the amount of water in each sample was determined and compared with the values measured by Karl-Fischer titration. The water contents in high vacuum pump oil (MR-100) (Neovac Corp. Japan), grapeseed oil, margarine and honey samples were measured in the same procedures.

RESULTS AND DISCUSSION Probe 1 was synthesized according to previously reported procedures.17 The UV/Vis absorption and emission spectra of probe 1 in THF displayed red absorption (λabsmax 590 nm) and emission (λemmax 618 nm) bands. There was no significant change in the absorption and emission maxima in solvents with different dielectric constants (ε) ranging from 1,4dioxane (ε = 2.2) to dimethyl sulfoxide (ε = 47.2) (Figures S1S2 and Table S1). The fluorescence quantum yield (ΦF) for probe 1 is modest, as expected for narrower bandgap dyes: ΦF (1) = 0.36 vs. ΦF (2) = 0.82, ΦF (3) = 0.80, ΦF (4) = 0.61 (Table 1). In highly polar solvent such as DMSO, the emission of probe 1 became very weak (ΦF,DMSO 0.05) (Table S1). The sensory response of probe 1 toward water (0−99%, v/v) was first explored in THF at 25 oC (Figure 1). Upon addition of aliquots of water, the red-shifted absorption and emission bands of 1 became less intense as emerged new blue-

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shifted absorption and emission peaks at 517 nm and 526 nm, respectively. The latter green emission was remarkably enhanced, and resembled the photophysical properties of its mesoalkyl counterparts 2−4. These dramatic spectral changes to shorter wavelengths (∆λabs = 73 nm; ∆λem = 92 nm) are consistent with changes in the electronic nature of the mesosubstituent upon the nucleophilic addition of water to the aldehyde group of probe 1 and the formation of an aldehyde hydrate 1⋅⋅(H2O). Table 1. Photophysical properties of compounds in THF. λabsmax [nm]

ΦF

[M cm ]

λemmax [nm]a

Probe 1

590

35000

618

0.36b

2

504

82000

525

0.82c

3

505

81000

526

0.80c

4

512

71000

527

0.61c

Compound

ε -1

-1

a Excited at 460 nm for 2-4 and 550 nm for probe 1. bQuantum yields relative to cresyl violet in EtOH (ΦF = 0.56). cQuantum yields relative to fluorescein in 0.1 N NaOH (ΦF = 0.95).

Figure 1. (A) Absorption and (B) emission spectral changes of probe 1 (10 µM) in pure THF containing different amounts of water (0–99%, v/v) at 25 oC. The spectra were obtained immediately after the addition of probe 1 to each solution. (C) Absorbance at 517 nm (ε517) as a function of [water] (0–99%, v/v) in THF. Inset: Linear correlation between ε517 and [water] (0–15%, v/v). (D) Relative fluorescence intensity at 526 nm as a function of [water] (0–99%, v/v) in THF. Inset: Linear correlation between F/F0 at 526 nm and [water] (0–20%, v/v). λex = 460 nm. F and F0 correspond to the fluorescence intensity in the presence and absence of water in each solution, respectively. Error bars represent the standard deviations, n=3. Some error bars are hidden by point markers.

The fluorescence intensity at 526 nm progressively increased together with water content from 0% to 40% where it reaches a plateau. When the water content reaches 20%, the emission intensity at 526 nm is approximately 280-fold higher than in pure THF (Figure 1D inset). While the increase in fluorescence intensity of the green emission at 526 nm is highly sensitive to the water content, the decrease in fluorescence intensity of the red emission at 618 nm is less affected (Figure 1B). Accordingly, as shown in Figure 2, the shift in the emis-

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sion profile resulted in dynamic color changes from red to orange to yellow to green under irradiation (365 nm) with a hand-held UV lamp, depending on the amount of water in THF. Note that all measurements could be performed right after the addition of probe 1 into THF−H2O solutions because the fluorescence enhancement at 526 nm was complete upon uniform mixing, which took about 10 seconds (Figure S29). Fluorescence intensity of the green emission at 526 nm was further enhanced by increasing pH (Figure S33).

Figure 2. Photographs of probe 1 in THF containing different amounts of water (left to right: 0, 0.1, 1, 4, 10, 20, 30%, v/v) at 25 °C under 365 nm UV light. Each photograph was taken immediately after the addition of probe 1 (10 µM) to each solution in a 96-well plate.

Subsequently, the sensing performance of probe 1 toward water (0−99%, v/v) was assessed in other common polar aprotic solvents at 25 oC: 1,4-dioxane, CH3CN, DMSO, and acetone (Figure 3 and Figures S15-S27). All showed similar emission spectral changes with slightly different sensitivities. As shown in Figure 3B, good linear relationships between fluorescence intensities at the green emission band and the water content were obtained in the range of 0−20% v/v, enabling the straightforward quantitative detection of water in various organic solvents. The detection limits (LOD) for water using probe 1 were determined to be 0.003% in CH3CN and THF, 0.006% in acetone, 0.007% in DMSO, and 0.008% in 1,4-dioxane, on a 3σ/slope basis (Figure S28 and Table S2). These LODs are comparable to previously reported values with other fluorescent probes.6a

Figure 3. (A) Changes in green fluorescence intensity (λem = 520–530 nm) of probe 1 (10 µM) in various organic solvents containing different amounts of water (0–99%, v/v) at 25 °C. The spectra were obtained immediately after the addition of probe 1 to each solution and fluorescence intensities of green emission (λem 526 nm (THF), 523 nm (acetone), 526 nm (CH3CN), 528 nm (1,4dioxane), 530 nm (DMSO)) were recorded. λex = 460 nm. (B) Plot of the relative fluorescence intensities (F/F0) of green emission vs. the amounts of water (0−20%, v/v) in various solvents. F and F0 correspond to the fluorescence intensity in the presence and absence of water in each solution, respectively.

In addition to the changes in the fluorescence emission of probe 1 in the presence of water, an isosbestic point was located at 533 nm in the absorbance spectra, accompanying a decline and a rise in absorbance at 590 nm and 517 nm (Figure 1A) upon increasing water concentration in THF (0−99%,

v/v). Therefore, a quantitative analysis of water could be also achieved by measuring the absorbance at 517 nm (ε517).18 A good linearity was found for this absorbance-based approach in the 0–15% water concentration range (Figure 1C, inset). The single isosbestic point observed at all probe 1/water ratios indicates that only one spectrally distinct hydrate adduct of probe 1 is formed. 1

H NMR and HPLC-MS analysis of probe 1 in water−THF solutions further support the sensing mechanism depicted in Scheme 1B. In 1H NMR experiments (Figure 4), the aldehyde proton (Ha) signal at 10.3 ppm in THF-d8 (Figure 4A) was diminished in a D2O−THF-d8 mixture (1:9, v/v, Figure 4B) and completely disappeared in D2O−THF-d8 mixture (1:1, v/v, Figure 4C). Its disappearance is accompanied by the growth of a new signal at 6.5 ppm, which is assigned to the d-proton (Hd) of aldehyde hydrate product 1⋅⋅(H2O). Formation of the aldehyde hydrate results in upfield shifts of the pyrrolic proton signals (Hb/Hc vs. Hb’/Hc’). These upfield shifts and the blueshifted absorption spectra of probe 1 in the presence of water are both consistent with the presence of an electron-donating group at the meso position. ESI-MS analysis showing a signal at 267.1 m/z [probe 1 + H2O + H]+ also provides evidence for the formation of the aldehyde hydrate 1⋅⋅(H2O) in aqueous solutions (Figure S39). Based on the 1 : 1 stoichiometry of aldehyde hydrate formation, the equilibrium constant (Kassoc) for the hydration of probe 1 was calculated to be 7.9 × 10-1 M-1 (Figure S30).

Figure 4. Partial 1H NMR spectra of probe 1 (20 mM) in THF-d8 containing different amounts of D2O, (A) 0%; (B) 10%; (C) 50% (v/v).

One limitation of this sensing scheme is the formation of hemiacetals of probe 1 with alcohols, which also leads to a fluorescence enhancement at 526 nm (Figures S34-35 and S38). Control experiments have also established that the presence of traces of dimethylamine in DMF solvent interferes with the assays (See Figure S36 for more details). Therefore the developed assays are not suitable for the determination of water in alcohol solvents19a or DMF. Aldehyde hydrate formation is particularly favourable for probe 1, but not observed for the closely related BODIPY dye, meso-formyl-1,3,5,7tetramethyl BODIPY (5, Figure S37), which is non-emissive in solution,15b,16 and showed negligible changes in its emission spectra upon the addition of water (50%, v/v) at 25 °C (Figure S37). These results underline the importance of steric effects

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in addition to electronic effects for the success of our sensing scheme. To simplify these water assays, probe 1-impregnated paper strips (10 mm x 10 mm) were prepared by the application of a THF solution (1 mM) and subsequent drying. The dry paper strips are non-emissive, which we attribute to the π−π stacking of these planar dyes in the solid state. To perform qualitative water analysis, one only has to deposit a drop of the solution to be analyzed onto the paper strip (Figure 5). Under UV irradiation, deposition of a drop of dry THF immediately results in a strong red emission (Figure 5A), whereas addition of a drop of THF−H2O solution reveals orange to green emission as a function of the water content in the sample (0.1−30% range). This can provide a useful ballpark figure for estimating water content in samples. The simplicity of this assay for water in organic solvents is comparable to that of the pH paper strips that are common staples in nearly every chemical laboratory.

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S3). The same agreement was observed in the measurement of low water levels in a hydrocarbon-based vacuum pump oil (probe 1: 0.025±0.008% water vs. K-F titration: 0.029% water), suggesting potential applications of probe 1 for water assays in real-life samples.19 Instant qualitative water detection based on emission color was also readily achieved by immersing the probe 1-loaded paper strip into solutions of the analytical samples in dry THF (Figure 5C).

CONCLUSION In summary, we studied probe 1 as a colorimetric and fluorescent indicator for the analysis of water in organic solvents, and real-life API and foodstuff samples. The reactivity of probe 1 is so far unmatched among fluorogenic water probes, allowing for instantaneous assays to be performed at room temperature. Its rapid response and obvious emission color changes are clear advantages over previously reported fluorescence-based water probes, making probe 1 attractive for on-site qualitative and quantitative analysis in practical applications. At this stage of development, water assays using probe 1 are most suitable for rapid estimates that could be supplemented by complementary analytical methods for samples judged to be of greater concern. The probe 1-loaded paper strips, which operate like common pH paper strips for the quick evaluation of a solution’s pH, are as particularly simple in this regard for the detection of water in quality control and production processes.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental details and additional spectroscopic data Figure 5. (A) Photographs of the paper strips (10 mm x 10 mm) impregnated with probe 1 (1 mM) immediately after dropping of water−THF mixture solutions (5 µL) containing different amounts of water (left to right: 0, 0.1, 1, 4, 10, 20, 30%, v/v) at 25 °C under UV irradiation (365 nm). (B) Fluorimetric determination (λex=460 nm; λem=526 nm) of the water content of vacuum pump oil, (O1; red), grapeseed oil (O2; pale red), API atorvastatin (A; orange), API tenefovir disoproxil (T; blue), margarine (M; pale green) and honey (H; green) dissolved in dry THF containing probe 1 (10 µM) at 25 °C, against reference standards (black circles). The water content in each sample was determined from the average of 4 independent measurements. (C) Visual determination of water contents in vacuum pump oil, (Oil-1), grapeseed oil (Oil2), API atorvastatin (drug A), margarine and honey dissolved in dry THF, using the probe 1 paper strips under UV irradiation (365 nm). All photographs were taken after dropping the paper strips into vials containing the sample solution and then stirring for 10 seconds.

Quantitative analysis of water content in active pharmaceutical ingredients (API) was demonstrated for the antihyperlipidemic agent atorvastatin calcium trihydrate (A) and the antiretroviral prodrug tenofovir disoproxil fumarate (T). The API powders were dissolved in dry THF (10 mg/mL) containing probe 1 (10 µM), and assayed fluorimetrically. The result of these assays (4.81±0.22% water in A and 5.20±0.21% water in T) were found to be in agreement with the results obtained by K-F titration (A: 4.80%; T: 5.06%). Similarly, the water content of foodstuffs such as grapeseed oil, margarine and honey measured with probe 1 were also in good agreement with values determined by K-F titration analysis (Figure 5B and Table

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Y.K.). Fax: +82-2-961-0443.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This research was supported by National Research Foundation of Korea (NRF) grants funded by the Korean government (MSIP) (NRF-2015R1A2A2A01004632 and NRF-2015R1A5A1008958). We thank Sungwun Pharmacopia for API samples.

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