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Detection of Moisture by Fluorescent OFF-ON Sensor in Organic Solvents and Raw Food Products Pawan Kumar, Rahul Kaushik, Amrita Ghosh, and D. Amilan Jose Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b03949 • Publication Date (Web): 10 Nov 2016 Downloaded from http://pubs.acs.org on November 10, 2016
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Analytical Chemistry
Detection of Moisture by Fluorescent OFF-ON Sensor in Organic Solvents and Raw Food Products Pawan Kumar, Rahul Kaushik, Amrita Ghosh and D. Amilan Jose* Department of Chemistry, NIT-Kurukshetra, Kurukshetra, Haryana, India. Email:
[email protected] ABSTRACT: Copper complex based new class of fluorescence OFF-ON sensor 1.Cu has been reported for the detection of a trace amount of water in various organic solvents such as CH3OH, THF, CH3CN and acetone by means of fluorescence emission intensity. The probe is highly responsive to water in THF (DL = 0.003 wt %). The dissociation of copper from probe 1.Cu in presence of water is responsible for the fluorescence change and it was confirmed by ESI-MS, 1H-NMR and fluorescence life time studies. Real application of the probe was successfully applied for the detection of moisture content in commercial products such as salt, sugar, wheat and washing powder.
Water is one of the most important and essential substance not only in daily human life but also in Chemistry. It is an important impurity that needs to be removed to perform many chemical and industrial production processes. For many sensitive chemical reactions water controls the yield and selectivity of the final product. Traditional analytical techniques such as Karl Fischer titration, chromatography and other spectroscopic techniques are available for the analysis of water contents in organic solvents.1 The Karl-Fischer method permits detection of water in ppm level but it has disadvantage of using toxic reagents (i.e., CH3OH, I2 and SO2). Therefore simple, efficient and convenient methods are required for the estimation of water in regular laboratory works such as organic synthesis, solvent purification, liquid chromatography and industrial processes2,3. Chemical sensors find an extensive range of applications in food industry and material based research. Therefore, colorimetric and fluorescent sensors may be good choice for the detection of water, particularly sensing of moisture in organic solvents and raw food materials. Universal solvent water may cause several characteristic interactions with excited molecules due to its large dielectric constant and proton donating and accepting properties. Hence, number of fluorescent probes for the detection of trace amount of water has been developed so far4-12. However, these systems have drawbacks such as fluorescent turn off, less sensitivity and slower response time; these features makes it difficult to detect a trace amount of water in the system. Herein, we report dansyl based non-fluorescent metal complex 1.Cu for the detection of trace amount of water in organic solvents and some commercial products. We have selected dansyl molecule because it is an excellent molecule for ICT-based sensors, owing to its unique donor–acceptor behavior. Synthesis of compound 1 and 1.Cu (Scheme 1) has been achieved in good yields and characterized by standard analytical methods (See ESI). UV-vis absorption spectrum (Figure S1) of 1 displayed two distinct bands at 256 and 341 nm. The band at 256 nm was attributed to a charge transfer (CT) transition involv-
ing N-amine as a donor and the dansyl moiety as an acceptor.
Scheme 1: Structures of receptor 1 and 1.Cu The other band at 341 nm was ascribed to a dansylbased CT transition. Compound 1 displayed strong emission band at λmax of 531 nm (λext of 345 nm) in CH3CN (Figure S1). Fluorescent intensity of 1 at 531 nm was completely quenched upon binding to Cu2+ (Figure S1). This quenching was due to the binding of Cu2+ centre (d9) to the DPA unit and paramagnetic nature. For the copper complex (1.Cu), two UV-vis absorption ban.ds appeared at 259 and 335 nm, respectively (Figure S1). The binding stoichiometry (1:1) and binding constant were evaluated from fluorescence titration of 1 with Cu(ClO4)2.6H2O in CH3CN and CH3CN/H2O (Figure S2-S3). Derivatives of dansyl (1-(dimethylamino)-naphthalene5-sulfonyl) compounds are known for the twisted intramolecular charge transfer (TICT) behavior13,14, in which dimethylamino moiety functions as a donor part and naphthalene sulfonyl as an acceptor part. TICT is highly dependent upon solvent polarity indicated by change in fluorescence band. Therefore, we have measured the fluorescence spectra of 1 and 1.Cu in a variety of solvents from low polarity to high polarity and the spectral data were presented in Table S1 and Table S2. The fluorescence spectrum of 1 and 1.Cu shows a large Stokes shift with increasing solvent polarity (Figure S4-S5). The change of calculated relative quantum yield (Φf) with respect to solvent polarity (Table S1) is ascribed to a strong effect of
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the solvent in the deactivation of excited state of the 1 and 1.Cu. Lippert–Mataga equation could be used to estimate solvent effects on the fluorescence spectra, which is basically a plot of the Stokes shifts versus the solvent polarity. For probes 1 and 1.Cu the Stokes shifts increase with the solvent polarity, but the shift generated significant deviations from the correlations as shown in (Figure S6-S7). The poor linearity plot may be due to the fact that Lippert–Mataga method does not account for the hydrogen bonding to the fluorophore or internal charge transfer15.
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(wt %) for all the four solvents (Figure 2). When the water content ranges between 0.1 to 3.6 wt % the fluorescence intensity increased dramatically. It can be clearly seen that the emission intensity linearly increased with wt % of water content and the plots for these solvents fit in straight lines passing through the origin, which are required for the practical use of the fluorescent sensor for water (Figure S8).
Figure 3: (A) Fluorescence ON-OFF behaviour of probe 1 and 1.Cu in CH3CN. (B) Fluorescence OFF-ON behaviour of probe 1.Cu as a function of water in CH3OH, Acetone, CH3CN and THF at λex = 345 nm.
Figure 1: Emission titration as a function of water content in −5 CH3OH, Acetone, CH3CN and THF [1.Cu] = 2.5 × 10 M, λex = 345 nm.
However, the observed large Stokes shift of 1 in different solvents might be due to the formation of a twisted intramolecular charge transfer (TICT) state. The emission maximum of 1.Cu was changed from 375 nm in ethyl acetate to 536 nm in CH3OH indicates the charge transfer nature of the emitting state. The emission red shift with increasing solvent polarity was a typical phenomenon for an emission from a TICT state as demonstrated by the large dipole moment in the excited state.
Figure 2: Plot of 1.Cu fluorescence intensity in CH3OH, Acetone, CH3CN and THF with increasing water content. [1.Cu] −5 = 2.5 × 10 M, λex = 345 nm.
This observation motivated us to check the effect of water. Thus, we have measured fluorescence spectra of 1.Cu in MeOH, THF, CH3CN and acetone containing various concentrations of water. As shown in figure 1, the corresponding fluorescence spectra of 1.Cu exhibited significant changes in intensity with a negligible change in spectral position. The change in emission intensity after addition of water was plotted against the water concentration
The fluorescence OFF-ON behaviour of 1.Cu in presence of water in various solvent at 2.5 x10-5 M concentration can be observed through naked eye under UV light chamber (Figure 3). We have calculated the Detection limit (DL) and Quantitation limit (QL) based on the equations DL = 3 x σ /ms and QL = 10 x σ / ms, where σ is the standard deviation of the blank samples and ms is the slope of the calibration curve. The DL (wt %) were found to be 0.00302, 0.033, 0.052, 0.448 and QL (wt %) were found to be 0.01, 0.109, 0.175 and 1.493 for THF, CH3OH, acetone and CH3CN respectively. Water detection in THF is 100 times higher as compared to CH3CN, and 10 times higher as compared to acetone or CH3OH. The solvent order for water selectivity is THF> CH3OH > acetone> CH3CN. The emission of 1.Cu solution in THF enhanced in the presence of water with as low as 0.003 (wt %) concentration. 1.Cu responds similarly in the presence of CH3OH, Acetone and acetonitrile with higher than 0.03 wt % of water, still the enhancement is much lower than that in the presence of water 0.003 wt % in THF. This describes a threshold that brings about 1.Cu specificity towards water molecules in THF. So in principle it is possible to differentiate between fresh anhydrous THF and older wet THF simply by recording an emission spectrum of known concentration of 1.Cu with THF. The calibration curves for the determination of water in different solvents were obtained as follows CH3CN, F = 1.936 [H2O] + (- 0.0089) R2 = 0.99734, [H2O] = 0 – 0.30 wt %
(1)
Acetone F = 6.459 [H2O] + (0.0127) R2 = 0.99963, [H2O] = 0 – 0.50 wt %
(2)
THF F = 428.147 [H2O] + (- 10.1066) R2 = 0.99436, [H2O] = 0 – 0.60 wt %
(3)
CH3OH, F = 33.180 [H2O] + (0.0089) R2 = 0.99999, [H2O] = 0 – 0.50 wt %
(4)
Where, F stands for fluorescence peak intensity. The absolute magnitude of the slope is larger for THF, fol-
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lowed by CH3OH, then acetone and lastly CH3CN. These results indicated that the probe 1.Cu is successful in the detection of water in different solvents. Furthermore, the interference of water detection with other solvents such as acetone, CH3CN, CH3OH to the THF solution of 1.Cu in presence of 0.89 wt % of water were checked. As shown in figure S9-S10 the fluorescence intensity was not affected much, which is indicative of high sensitivity 1.Cu for water detection. However we found that CH3OH has some effect on probe 1.Cu. Upon addition of more than 13 wt % of CH3OH in THF solution of 1.Cu displayed small change in fluorescence intensity (Figure S11), but the changes were very small and almost neglible. The fluorescence spectra of 1 in CH3CN that contained various concentrations of water displayed negligible change or no enhancement in the emission intensity even after addition of 5 wt % of water (Figure S12). This result confirms the role of copper metal center in the detection of water in organic solvents. Therefore the mechanism of the fluorescence enhancement of 1.Cu is may be explained due to partial or complete displacement of copper from 1.Cu in presence of water molecules. Similar observation of change in fluorescence intensity with water for copper complex was also observed by others16. To confirm this we have calculated the association constant of 1 with Cu2+ in CH3CN-Water (80/20, v/v) mixture, the calculated association constant by the Hill plot is 1.2 x 104 M-1, which is nearly five times lesser than the binding constant 6.28 x 104 M-1 determined by using only CH3CN as a solvent. This result further confirms the weak association of copper with ligand in presence of water. This may lead to the displacement of copper upon addition of water. The m/z signals in the MS-ES+ spectra were observed at 433.0 (100 %) and 495.4 (28 %) for 1.Cu in the presence of CH3OH water mixture, here the signal at 433.0 corresponds to the ligand 1 and signal at 495.4 corresponds to the 1.Cu (Figure S13). This data also supported the dissociation of copper from 1.Cu in presence of water in CH3OH. Fluorescent lifetime measurements displayed a substantial decrease in the lifetime of free ligand 1 (τ0 = 12.9 ns, Φf = 0.600) on complexation with 1.Cu (τ0 = 1.2 ns, Φf = 0.179). But, upon addition of water to the 1.Cu in THF it regains its original lifetime (τ0 = 12.9 ns, Φf = 0.416) (Figure S14). The displacement of copper from 1.Cu in presence of water was checked by 1H-NMR experiments. As shown in figure S15, upon addition of water to the probe 1.Cu, the broad NMR peaks appeared due to paramagnetic nature of 1.Cu turned into clear signals corresponds to compound 1 only. This may be due to the displacement of copper from the probe. Life time study and NMR results also confirms the dissociation of Cu2+ from 1.Cu in presence of water. In order to investigate the response time, the time dependent responses of the probe were measured by exposed with water (0.74 wt %) in THF, CH3CN and Acetone. The result reveals that time response for the detection of water is quick and fast (Figure S16). Effect of pH on 1.Cu with and without water was studied at different pH ranges from 2 to 14.0. These results show that the response to water wasn’t affected in the pH range from 2.0
to 11.0. But at higher pH, it is expected that Cu2+ ion may precipitate and consequently at higher pH (>11) the probe may not be suitable for water detection (Figure S17). Further, the Temperature effects on moisture detection were studied in THF from 0-65°C by monitoring emission intensity at 505 nm. The emission intensity of 1.Cu with water increases as temperature increases (Figure S18). But in absence of water it almost remains constant. These results indicate that the fluorescence response of water is sensitive even above and below RT. The results are compared with the previously reported fluorescence water sensors that comprises metal center, displacement approach or that follows ICT mechanism. As shown in Table 1 the current fluorescence sensor system 1.Cu is successful in the detection of water in THF and different polar organic solvents. Table 1: Some of the related previously reported receptors and LOD (wt %) comparison in different solvent. Receptor
Solvent
LOD
Reference
Eu(HTTA)3Phen complex
EtOH
0.002
17
Eu(DAF)2(NO3)2 complex
MeCN
0.003
18
Dye–Acetate ensemble
0.037 0.071 0.170
12
Dye–Fluoride ensemble
MeCN THF MeCN
Dye–Fluoride ensemble
MeCN
0.160
20
4-morpholinyl-1,8naphthalimide
1,4Dioxane MeCN EtOH 1,4Dioxane MeCN EtOH 1,4Dioxane MeCN Acetone THF Acetone MeCN THF MeOH
0.008 0.006 0.015
21
0.019 0.038 0.060
22
0.049 0.021 0.016 0.020
23
0.052 0.448 0.003 0.033
This work
N-amino-4-(2hydroxyethylamino)-1,8naphthalimide N-Heteroaryl-1,8naphthalimide
1.Cu
19
Moisture is a critical component in the commercial products and raw food materials. Hence, determination of the moisture has become one of the most challenging areas in the food industry. It is important to know the moisture content of raw products during the mixing stage as water affects the quality and stability of the final product. Testing moisture in raw food materials will allow for the necessary adjustments to control the moisture levels. Therefore, it is important to have simple laboratory method for the detection of moisture in various food and commercials products. In this connection, we have tested our fluorescent OFF-ON probe 1.Cu, for the possible detection of moisture in various raw food materials such as sugar, salt, wheat and a commercial product washing powder. In a typical experiment 50 mg of raw food prod-
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ucts were added to the solution of 1.Cu (2.5 x 10-5 M) in dry THF or dry CH3CN and the emission intensity was measured after removing the particles by centrifugation and followed by filtration. The emission intensity obtained from above experiment was compared with the emission intensity observed from 50 mg of completely dried raw food products. The samples were oven dried at 100°C for 1 hr and then they were kept in desiccator to remove the moisture. The emission intensity (F/Fo) of dried and moisture absorbed samples were compared (Figure 4).
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switching OFF-ON emission intensities. Currently, we are attempting for better understanding of the water sensing mechanism and more sensitive detection of moisture in other commercial products. ASSOCIATED CONTENT Supporting Information Available: The following file is available free of charge. Final SI, the contents of which are: 1 Experimental section, UV-vis, Fluorescence, HNMR, fluorescence lifetime spectra for different studies are available in the Supporting Information. AUTHOR INFORMATION
Corresponding Author Email:
[email protected], Phone:+91-1744233559. Fax: +91-1744238350.
Notes: The authors declare no competing financial interests. ACKNOWLEDGMENT DAJ and AG acknowledge the financial support from the Department of Science and Technology, India, for the SERBDST project grant SB/FT/CS-195/2013 and SB/FT/CS-193/2013. REFERENCES
Figure 4: Changes in fluorescence intensities of 1.Cu in presence of various commercial products in dry THF [1.Cu] = 2.5 −5 × 10 M, λex = 345 nm.
As shown in figure 4, the moisture absorbed sample displayed considerable increase in emission intensity. Among four different commercial products we found that salt has more moisture content as compared to other two products. From the calibration curve we found that approximately 0.22 wt % of moisture observed in the salt sample (Figure S19-S23). In the dried samples, we have also spiked with around 0.5 wt % of water and emission spectrum was recorded, it shows increase in the emission intensity at 505 nm. These simple fluorescence studies confirm that 1.Cu has advantage for the detection of moisture in the commercial sample that is being used as important raw materials for the various food products. The presence of high moisture in the environment causes immense problems in Pharmaceutical, Fertilizer and Electronics Industries. We have also tested Probe 1.Cu for detection of moisture contents at four different surroundings spaces such as cemented cupboard, air conditioned room with dehumidifier, refrigerator and lavatory (Figure S24-S28). The change in fluorescence emission was monitored every 30 minutes. The experiments results suggest that air conditioned room with dehumidifier have less moisture content compared to other places. In conclusion, we have demonstrated that the detection of water in various organic solvents such as CH3OH, THF, CH3CN and acetone by means of fluorescence emission intensity using 1.Cu as a probe. The present receptor is highly responsive to water in THF (DL = 0.003 wt %). This OFF-ON sensor is rapid and convenient fluorescent method to analyze the anhydrous organic solvents before using them in water-sensitive reactions/reagents. This will be advantageous in improving the reaction yields and avoid the side products. Moisture content in commercial products was also determined by the probe 1.Cu by
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