Reversible Colorimetric Sensor for Moisture Detection in Organic

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A reversible colorimetric sensor for moisture detection in organic solvents and application in ink-less writing Pawan Kumar, Rahul Sakla, Amrita Ghosh, and D. Amilan Jose ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 07 Jul 2017 Downloaded from http://pubs.acs.org on July 7, 2017

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A reversible colorimetric sensor for moisture detection in organic solvents and application in ink-less writing Pawan Kumar, Rahul Sakla, Amrita Ghosh and D. Amilan Jose* Department of Chemistry, National Institute of Technology (NIT) Kurukshetra, Kurukshetra-136119, Haryana, India Email: [email protected]

KEYWORDS: Moisture sensor, Ink-less Writing, Sudan-III dye, Alizarin Red S, Reversible probe

ABSTRACT: Colorimetric sensors based on Sudan- III (1) and Alizarin Red S (2) have been developed for the detection of a trace amount of water in organic solvents such as THF, acetone, acetonitrile and DMSO. The deprotonated (anionic) forms of 1 and 2 such as 1.F and 2.F are re-protonated by using a trace amount of water. Deprotonation of 1 and 2 were obtained by using fluoride anion. Test papers of 1.F and 2.F in organic solvents with and without moisture showed dramatic changes in color. Receptor 1.F exhibits high sensitivity for water in acetone and THF with the detection limit as low as 0.0042 and 0.0058 wt %. Remarkably probes 1.F and 2.F are reversible in nature both in solution and test stripes. 1.F and 2.F are reversible and reusable for sensing moisture in the organic solvents with high selectivity, high sensitivity, and fast response. The reversible moisture sensor 1.F has also been used for the application in ink-less writing. anion were re-protonated by using a trace amount of water. We have established that it could be an excellent technique for Introduction the detection of water by naked eye and most importantly the Chemical sensors that change the optical signal on exposure to entire process is reusable/reversible in nature. trace amount of moisture or humidity are highly important in Result and discussion environmental monitoring, industrial production, food proPhenol and Catechols are known for the formation of the cessing and biomedical field. Determination and elimination strongest hydrogen bond with fluoride through F---O-H hyof water in routinely used organic solvents and reagents are drogen-bonding interactions.17-18 However, in the presence of crucial in synthetic chemistry and industrial methods. Conventional methods such as Karl Fischer titration, Gas Chromatoan excess of F¯, it forms the anionic form due to the deprotonation of –OH group caused by Brønsted acid-base graph, and other spectroscopic techniques are available for the type reaction.18 We have chosen the off-the-shelf commercial determination of trace amount of water in organic solvents.1 However, lack of continuous monitoring, use of toxic chemidyes 1 and 2 (Chart 1) containing phenol and catechol as anion cals and long experimental duration makes them not suitable binding sites. Compounds 1 and 2 also contain chromophore for all the cases. Electronic moisture sensors are also available subunits (acting as electron acceptors) whose electronic propfrom commercial sources, but they are less safe, expensive and erties were modified on interactions with electron donating have their own lifetime.2 Therefore, a low-cost, simple, rapid, fluoride anion. and highly sensitive portable device is required to detect tracelevel of water in organic solvents and fine chemicals. Many sensitive fluorescent based moisture sensors are available in literature.3-11 However, colorimetric detection is a simple and convenient method compared to the other traditional and fluorescent methods. Colorimetric moisture sensor expresses a color change in the presence of moisture which could be detected by the naked eye without any sophisticated analytical instrumentation facility. On the other hand, only a few coloriChart 1: Structures of the compound 1 and 2 used for moismetric sensors are available for the recognition of a trace ture sensor 12-16 amount of water in organic solvents. Most of the known colorimetric moisture detecting probes are expensive, not reUv-Vis absorption spectra of 1 and 2 with and without F¯ versible, demonstrate slower response time, need multistep were monitored in different solvents such as acetone, acetonisynthesis and less sensitive. Hence, it is very important to detrile, THF and DMSO as shown in Figure S1 and S2. Comvelop a highly sensitive, reversible and fast responding colorpound 1 (0.025 mM) shows a strong absorption band at 345imetric probe for the detection of moisture. 350 nm and 500–511 nm due to the azo chromophore. The In this paper, we report the use of two commercially availacompound 2 (0.15 mM) shows an absorption band at around ble off-the-shelf dyes sudan-III (1) and alizarin red S (2) for 430-432 nm due to n→π* transitions of the p-benzoquinone the naked eye detection of moisture in organic solvents rougroup (diketo form) condensed between two rings. tinely used for chemical synthesis. The deprotonated (anionic) forms of dyes 1 and 2 obtained by using highly basic fluoride

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Figure 1: Color changes of compounds 1 and 2 in acetone, CH3CN, DMSO and THF after the addition of [(nBu4N)]F¯ and their reversible color changes by the introduction of a trace amount of water in the solvents.

After the addition of the excess of F¯ ions to the solutions of compounds 1 and 2, prominent changes in the color (Figure 1) and in the features of the Uv-Vis absorption spectra were observed due to the deprotonation of phenol groups. For compound 1 (0.025 mM), the color change was observed in the presence of F¯ (0.22–1.42 mM) corresponds to a ratiometric decrease (at 500 nm) and increase (at ~603-630 nm) in absorbance with an isosbestic point at ~540-550 nm (Figure S3). For compound 2 (0.15 mM), the color change was monitored in the presence of F¯ (0.26 – 2.25 mM). In the absorption spectra, a ratiometric decrease of the soret band centered at 430 nm and increase at ~540-575 nm with two isosbestic points (at the range ~396-406 nm and ~468-481 nm) were found (Figure S4). The red shifts accompanied by the welldefined isosbestic point indicating the formation of a new anionic species for both the compounds. In the case of 2, the dianionic form has been confirmed by the absorption band around 550 nm with F¯ ion. The significant red shifts (for both 1 and 2) in the Uv-Vis spectra (Table 1) are due to extensive delocalization of electrons that reduces the required energy for transition and thus shifts the absorption band towards the longer wavelength. Table 1: Uv-Vis spectral data (in nm) for 1 and 2 with F¯ and water

1

2

Solvents

Blank

With F¯

Red Shift

With water

CH3CN THF DMSO Acetone CH3CN THF DMSO Acetone

345, 501 350, 503 349, 511 345, 501 430 432 430 432

346, 603 347, 628 349, 622 348, 617 353, 545 360, 576 356, 557 357, 545

102 125 111 116 115 144 127 113

345, 504 350, 503 349, 515 345,503 430 432 430 432

It is well known that in the presence of excess F¯ ion suitably substituted H-bond donor-receptor functionality undergoes deprotonation rather than the supramolecular interaction.19 The higher stability of the polynuclear aggregate HF2¯ contributes in assisting the deprotonation process.20 Interestingly, Uv-Vis

spectra for 1 and 2 recorded by using [(nBu4N)]OH is very similar to the Uv-Vis spectra of 1 and 2, with an excess of F¯ ion. This confirmed the deprotonation of 1 and 2 (Figure S5). Further, the deprotonation process is also confirmed by 19F NMR spectra of [(nBu4N)]F¯ recorded in the presence of compound 1 and 2. The peak appeared at around -145 ppm assigned to HF2¯ (Figure S6) aggregates, proved the deprotonation of the phenolic proton of 1 and 2 (Scheme 1) with an excess of fluoride ion.

Scheme 1: Proposed mechanism for the detection of water by compound 1.F and 2.F. These studies established the Brønsted acid-base reaction mechanism existing in 1 and 2 with an excess of F¯. The deprotonation phenomenon on binding with F¯ ion occurred only with aprotic polar organic solvents such as acetone, DMSO, THF and acetonitrile. Nevertheless, the deprotonation could not be achieved with protic solvents such as water, methanol, and ethanol due to effective solvation of anionic species. The binding of F¯ in an aqueous medium by neutral sensor molecules are very rare due to the high solvation of F¯ in water. Even a trace amount of water would be enough to inhibit the binding of F¯ or to reverse the F¯– receptor interaction. This has prompted us, to check the effect of moisture/water with deprotonated species of 1 and 2 such as 1.F and 2.F (1.F = 1 with 9–66 equivalents of F¯and 2.F = 2 with 1.8–15 equivalents of F¯) in a different solvent medium. To our delight after addition of a trace amount of water to the solution of 1.F and 2.F in acetone, acetonitrile, DMSO and

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THF, the original color of 1 and 2 reappeared (Figure 1). This phenomenon suggested that in the presence of a trace amount of water or moisture, the original compounds were regenerated through reversible reaction as shown in Scheme 1.

0.0850% and 0.5592 % in acetone, acetonitrile, THF, and DMSO respectively.

(a)

(b)

Figure 2: (a) Uv-Vis titration for 1.F in acetone with 0-1.52 wt % of water (b) Uv-Vis titration for 1.F in acetonitrile with 0- 0.86 wt % of water content (c) Uv-Vis titration for 1.F in THF with 0-97 % of water content (d) Uv-Vis titration for 1.F in DMSO with 02.52 wt % of water content.

The sensitivity of the probes for the detection of water was determined by the systematic Uv-Vis absorption titration (Figure 2). The Uv-Vis absorption spectra of 1.F and 2.F have been investigated in THF, acetonitrile, acetone and DMSO containing various concentrations (wt%) of water. After the addition of water to the anhydrous THF/acetonitrile/acetone/DMSO solution of 1.F and 2.F, the corresponding original spectra of 1 and 2 reappeared (Figure 2 and Figure S7). Simultaneously, the color of the solution restored to the original red color for 1 and yellow color for 2 (Figure 1). These changes were owing to the regeneration of 1 and 2 through the protonation of the deprotonated species, 1.F and 2.F by water molecules (Figure 1). The change in absorption maximum after addition of water was plotted against the water concentration (wt%) for all the cases. The absorption spectra were dramatically changed when the water content ranges between 0.1 to 2.0 wt% for 1.F and 0.1 to 8 wt % for 2.F (Figure 3a and 3b). In the presence of moisture, the colorimetric changes in different solvents of 1.F and 2.F could be observed through naked eye without using any instrument or UV light chamber (Figure 1). The change in absorption by increasing the water content is linear. For the practical use of moisture sensor, the linear increment of change in absorption with an increase in wt% of water is required. Based on the plot the change in absorbance vs wt% of water (Figure S8 and Figure S9), the detection limits (DL) have been calculated by using the equations. DL = 3 x σ /ms. Where σ is the standard deviation of the blank samples and ms is the slope of the calibration curve. The DL of water for 1.F were found to be 0.0042 %, 0.0119 %, 0.0058 % and 0.0299 % in acetone, acetonitrile, THF, and DMSO respectively. For 2.F, the detection limits are 0.0221%, 0.0498 %,

Figure 3: Plot of change in absorbance as a function of the % water for (a) 1.F and (b) 2.F in acetone, acetonitrile, THF, and DMSO.

From the DL values we can assume that 1.F is very responsive to moisture in acetone and THF. Whereas, 2.F is very sensitive to moisture in acetone and acetonitrile, consequently 2.F could recognize wet acetone and acetonitrile by simple color change. The calibration curves for the determination of water in different solvents were obtained as follows: For 1.F CH3CN, A = 0.8847 [H2O] + (0.0165) R2 = 0.9512, [H2O] = 0 – 0.2911 v/v % Acetone, A = 0.5087 [H2O] + (-0.0059) R2 = 0.9945, [H2O] = 0 − 0.6820 v/v % THF, A = 1.089 [H2O] + (−0.0079) R2 = 0.9902, [H2O] = 0 − 0.3896 v/v % DMSO, A = 0.2123 [H2O] + (0.0025) R2 = 0.9940, [H2O] = 0 −0.4773 v/v % For 2.F CH3CN, A= 0.2116 [H2O] + (0.0057) R2 = 0.9893, [H2O] = 0 – 0.7597 v/v % Acetone, A = 0.2067 [H2O] + (0.0063) R2 = 0.9920, [H2O] = 0 – 0.6789 v/v % THF, A= 0.1078 [H2O] + (−0.0403) R2 = 0.9912, [H2O] = 0 – 1.3954 v/v % DMSO, A = 0.0135 [H2O] + (0.0094) 2 R = 0.9355, [H2O] = 0 – 7.2235 v/v % Where A stands for absorbance peak intensity

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For receptor 1.F the absolute magnitude of the slope is larger for THF, followed by acetonitrile then acetone and lastly DMSO. However, in the case of receptor 2.F, the absolute magnitude of the slope is larger for acetonitrile, followed by acetone, THF and DMSO. These results indicate that the probes 1.F and 2.F are successful for the effective detection of water in different solvents acetone, acetonitrile, and THF. Time-dependent responses of 1.F and 2.F with moisture were measured by exposing the solvent (acetone) with water. The results show the fast response time for the detection of water in each organic solvent (Figure S10). Further, a series of 1.F and 2.F solutions were prepared by using different wt% water spiked solvents. As expected blue-to-reddish brown color transition was observed (Figure S11). This quantitative analysis may be helpful to determine the extent of moisture concentration in an organic solvent by the simple color change.

Reversibility An ideal sensor should be reversible in nature to provide realtime measurements. Therefore, the important feature of any sensor is the reusability- so that it could be used for several times in a real life application. The reversible character of 1.F for water sensing was evaluated by using a different drying agent such as calcium sulphate, magnesium sulphate, sodium sulphate and molecular sieves to remove the externally added water molecules from the solution of acetone, acetonitrile, DMSO and THF. We found that molecular sieves (4Å pore size) are very effective to remove or absorb the water molecules from the solutions of 1.F. Molecular sieves are well known and widely used drying agent and it has the advantage of being chemically inert and stable.

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sieves. However, on the addition of water (1.739 wt %) to the same solution again or keeping the solution in moisture place, the absorption band at 503 nm is immediately reappeared and the color of the solution turns to red. This reversible process (Figure 4) was repeated for many times with an excellent cyclic color switching behaviour. The result from this entire procedure, confirmed the reversible nature of the sensor 1.F. To check the reversibility the similar experimental method was also carried out for 2.F. The results suggested that the probe 2.F is reversible but it could be used only for few cycles (Figure S12). The reversible nature of 1.F established a promising chemical sensor for moisture detection and for various applications.

Effect of temperature The effect of temperature on water/moisture sensor is important; because the level of moisture or water content may fluctuate at different temperatures. Therefore, the temperature effects on moisture detection were studied in acetone for 1.F and 2.F. The temperature effect was monitored from 5.055.0°C in the Uv-Vis absorption spectra at 503 nm and 430 nm wavelengths for both the compounds respectively. As shown in Figure 5, the trend of ratiometric change in absorbance was not affected by the change in temperature. It is also evident that the moisture detection is more effective at a higher temperature. These results suggested that the sensor 1.F could detect the water molecules in the temperature range of 10-55°C (Figure 5) and 2.F could do the similar job in the temperature range of 25-55°C (Figure S13)

Figure 5: Change in absorbance of 1.F (1.647 mM) with 1.765 wt% water at different temperature (5°C - 55°C) in acetone.

Portable analytical device:

Figure 4: Change in absorbance of 1.F at 618 nm in acetone upon the alternate addition of water and molecular sieves at room temperature.

As shown in Figure 4, the molecular sieves (approx 3 gm in 10 ml of solution) were added to the solution of acetone which contains 1.F and water mixture. The Uv-Vis absorption spectrum (618 nm) showed the original spectrum of 1.F and the color of the solution also restored from red to blue within few minutes. This observation indicates the removal of water molecules from the 1.F-water mixture by introducing molecular

Naked eye detection by a simple portable analytical device such as paper or thin film-based materials is the simplest technology because of handy, disposable and easy to carry nature. Hence, we have explored the possibilities of using 1.F in the development of test paper for moisture detection in different organic solvents. As shown in Figure 6a, test papers incorporated with 1.F were prepared by immersing 3 mM solution of 1.F in cellulose based Whatman filter paper. Subsequently, they were allowed to dry at room temperature for overnight in absence of moisture. These colored test papers were further used for on‐site naked‐eye detection of moisture in different solvents. The blue colored test papers (in the case of 1.F) change to the reddish-brown when it was dipped into the wet solvents such as THF, acetone, acetonitrile, and

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DMSO. However, almost no change in color was observed with dry solvents. Further, a series of test papers were prepared by dipping 1.F incorporated stripes into solvents with different wt% of water content. As anticipated, by introducing different wt% of water the color of the test papers transforms gradually from blue-to-red (Figure 6B). The intensity of the color change depends on the wt% of moisture content in all the solvents. Based on this result, a quantitative analysis of an unknown moisture concentration in organic solvents may be achievable. These experiments confirmed that the probe 1.F integrated paper-based analytical devices were efficient enough to detect moisture in organic solvents by simple dip test. The entire detection process was also very rapid. So in principle, it is possible to find out the percentage of water/moisture in the wet solvents such as THF, acetone, DMSO and acetonitrile before proceeding to any moisture sensitive reaction. Similarly, the test papers prepared from 2.F were also used for the detection of moisture in organic solvents (Figure S14).

were heated through a hot air gun and the color of the test papers changed from reddish brown to the original blue within few seconds. Moreover, the test papers could be re-used for several times (Figure 7) due to this reversible nature. Hence, the probe 1.F is reversible and reusable for sensing moisture in the organic solvents with high selectivity, high sensitivity and fast response.

Applications for ink-less writing in paper Recently, ink-less printing has attracted much attention because of the less environmental impact. Ink-less technology is a new revolutionary step in the history of printing technologies. Similar to ink-less printing, ink-less writing is also important in terms of environmental pollution. The inks used for writing, contain toxic and harmful chemicals; they are not safe for human health and environment. The information written by ink in the paper is permanent and most of the time it can’t be erased; this creates vast problems of paper waste and forest conservation. Therefore, it is important to search a green solution for the replacement of ink. (A)

(B)

Figure 8: (A) Rewritable (write and erase) text cycles carried out by ink-less water pen on paper incorporated with 1.F and the text erased by the temperature at 50°C. (B) Inkless text paper stored for a week under moisture-free condition.

Figure 6: (A) Color changes of test paper strips incorporated with 1.F in the presence dry and moisturised organic solvents such as THF, CH3CN, DMSO and acetone. (B) Test paper images after being exposed with different wt% of water spiked solvents.

Figure 7: Reversibility of portable test paper strip (color change from reddish brown to blue) incorporated with 1.F in the presence of moisture and followed by heat.

The test papers used for the detection of moisture in the solvent were also reversible in nature. The wet test papers

It is appealing to use water as ink for writing or printing on papers. The excellent reversible color changing properties of 1.F with water in test papers inspired us to use water as ink to the re-writable paper. As a result, a fountain pen filled with water was used to write a text in the normal notebook paper incorporated with 1.F (supporting information; section 2.6). As shown in Figure 8, water pen effectively used for writing text in the paper, here the color change helped to display the text in the paper. Importantly, the text written by using water is completely erased by heating the paper at 50°C for 30 seconds. The same paper could be used again for the writing and this process might be continued many times (Figure 8A). Repeated “write and erase” cycles were carried out with water ink pen. As shown in Figure 8, paper incorporated 1.F could be used multiple times without lowering the quality of the paper. Importantly, the text written on the paper was stable for several days if stored in an air-tight box and it was also rewritable (Figure 8B). This ink-less water pen with rewritable paper is a promising example; it could serve as an eco-friendly tool to meet the increasingly global needs for environmental protection. These experiments demonstrated that beyond the sens-

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ing of moisture, the probe 1.F also have potential applications for ink-less writing in the rewritable paper.

Conclusion: In summary, we have reported the use of off-the-shelf commercial dyes 1 and 2 for the detection of trace amounts of water in the organic solvents. Protonation of 1.F and 2.F using moisture (water) is the main working principle behind the colorimetric detection. Both the sensors have high sensitivity for moisture in THF, acetone, and acetonitrile. Test papers incorporated with 1.F are fast, reversible, reusable and portable moisture sensor that could measure moisture without the help of any instrument. In addition to the sensitivity and reversibility, these two probes have other advantages such as low cost, no chemical synthesis, easy availability, naked eye detection and fast response time. The test paper based portable analytical device is well explored for the rapid detection of moisture in organic solvents. Importantly, the ink-less writing in the paper with write and erase properties are the significant applications. We expect that the similar type of moisture detection mechanism has the immense potential for the advance of water sensor and ink-less writing.

ASSOCIATED CONTENT Supporting Information. General procedure for spectroscopic measurements, Uv-Visible absorption studies, 19F-NMR and color change images are available in the supporting information. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author Email: [email protected]

Author Contributions The manuscript was written through contributions of all authors.

ACKNOWLEDGMENT The author DAJ wishes to thank DST grant no.SB/FT/CS195/2013 and CSIR-EMR(II) grant no 01(2855)/16 for financial support. AG wishes to thank DST grant no.SB/FT/CS-193/2013 for financial support. RS acknowledges DST-Haryana for a HSCST research fellowship. The authors acknowledge Department of Chemistry, NIT-Kurukshetra for the lab facilities.

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