Gandhi Sivaraman*,
Balasubramanian Vidya and
Duraisamy Chellappa*
School of chemistry, Madurai Kamaraj University, Madurai-625021, India. E-mail: raman474@gmail.com; dcmku123@gmail.com
First published on 2nd July 2014
A rhodamine based derivative RDD-1 bearing dimethylaminobenzaldehyde has been designed and synthesized. It is a highly sensitive and selective chemosensor towards picric acid without having any interference from other NAC's. To the best of our knowledge, the present probe is the first example of a rhodamine based sensor that facilitates the detection of powerful explosive picric acid even at picomolar concentration.
During addition of PA, an absorption peak emerged at 535 nm. This absorption change in visible region makes feasible the colorimetric detection of PA by conspicuous colour change from colourless to pink. The appearance of a new absorption band at 535 nm is attributable to its spiroring-opened nature of the probe RDD-1. To check the selectivity, the photonics of Probe RDD-1 were investigated in the presence of few aromatic and nonaromatic compounds such as TNT, TNB, nitrobenzene, 2,4-dinitrophenol, benzoquinone etc. by UV-vis spectroscopy (Fig. 1).
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Fig. 1 UV-visible absorption responses of RDD-1(5 μM) toward different concentrations of PA in water (0–10 equivalents). |
The absence of colour change reveals that the probe RDD-1 is selectively sensing PA. Although strong acids such as TFA, HCl produced an absorbance change at 527 nm but PA addition leads to band at 535 and 425 nm. The change in the absorbance wavelength makes feasible to distinguish PA from other strong acids (Fig. S11†).
The fluorescence spectrum of RDD-1 (1 μM) exhibits weak emission band at 541 nm when excited at 480 nm. The addition of PA to the solution of RDD-1 results in appearance of fluorescence emission at 557 nm with high intensity along with the concomitant decrease of fluorescence at 541 nm (Fig. 2 and 3). A linear relationship was obtained when a plot of fluorescence intensity vs. concentration of picric acid was constructed. It provides that the probe RDD-1 will be applicable for detection of picric acid (Fig. S7†). The RDD-1, when monitored with the addition of compounds like TNT, TNB, nitrobenzene, 2,4-dinitrophenol, benzoquinone etc., did not exhibit any change in its photonics.
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Fig. 2 Fluorescence spectra of RDD-1 in the presence of various NAC's (PA, BQ, NB, NT, NP, DNB, TNT, DNP) in aqueous solution (excitation at 480 nm, slit width = 5 nm/5 nm). |
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Fig. 3 Fluorescence responses of RDD-1 (5 μM) toward different concentrations of PA in aqueous (0–10 equivalents) solution. |
This, in turn, ascertains the selective fluorimetric response of RDD-1 towards picric acid. To get an insight into the turn-on fluorescence recognition mechanism, the absorption and fluorescence spectral behaviours of the probe with the addition of trifluoroacetic acid (TFA) were analysed under identical conditions. Upon addition of TFA, an absorption maximum was observed at 527 nm whereas fluorescence was noticed at 547 nm. With PA addition two absorption maxima, one at 535 nm and another at 425 nm were observed while fluorescence was seen at 557 nm. Further the intensity of the peaks arising in both absorption and fluorescence spectra of RDD-1 by PA addition is higher compared to that surfacing with TFA addition. It is obvious that RDD-1 displays varied responses towards TFA and PA. This in turn advocates that the mechanisms involved in sensing TFA and PA by RDD-1 are different. It is envisaged that TFA being strong acid protonates the dimethylamino group of RDD-1. This protonation in turn triggers the opening of spirolactam ring to result in the observed photonic changes. It is noted that strong acids such as TFA, HCl and PA triggered different spectral changes for Rhodamine derivative, i.e., TFA, HCl induced a fluorescence at 547 nm, but PA showed ratiometric fluorescence enhancement at 557 nm. (Fig. S11 and S12 in ESI†) Exhibition of different photonic changes by RDD-1 towards PA sensing coupled with the observation of higher intense peaks designates that RDD-1 may associate with PA through intermolecular hydrogen bonding. This hydrogen bonding association between PA and RDD-1 is augmented by the observation of a peak in ESI-MS at m/z = 789.4 that corresponds to [RDD-1 + PA + H]+ (Fig. S6 in ESI†) and further supported Job's plot analysis (Fig. S5 in ESI†). Further to confirm the formation of picrate adduct, 1H-NMR study was undertaken. 1H-NMR of RDD-1 gives only a single peak corresponding to two methyl groups of dimethylamino group. 1H-NMR titration of RDD-1 with PA showed that these methyl groups at 2.83 ppm get shifted, and new peak at 8.6 ppm is observed upon the addition of PA which is characteristic of picric acid proton. Concomitantly in the 1H-NMR titration of RDD-1 with PA in CDCl3, the characteristic methyl peak of rhodamine spirolactam at 1.8 gets shifted and the NH broad band at 3.56 completely disappeared on subsequent addition of PA. These results further confirmed that the turn-on fluorescence response of RDD-1 to PA is ascribed to the opening of spirolactam and formation of picrate adduct (Fig. 4).
We also carried out pH studies and found that emission spectra of RDD-1 in aqueous solution. Further, the effect of PA added with RDD-1in different pH values was also studied. At acidic pH (1–4) the probe RDD-1 shows fluorescence which corresponds to the ring opened species and with the addition of PA to RDD-1, the emission intensity was not much affected. In the acidic pH range the emission intensity of PA added RDD-1 get decreased because of protonation of the RDD-1 in acidic pH leads to the fluorescence change. Hence PA detection by RDD-1 is affected by some extent in highly acidic pH (Fig. S13 in ESI†). The quick response of the probe RDD-1 with PA revealed from the fluorescence measurements in different time intervals (Fig. S8 ESI†).
To find if there is any interference in the detection of PA, the photonics of RDD-1 were monitored with the mixture of interfering compounds and PA. The competitive experiments clearly establish that there is no interference from the added components towards sensing PA. We also studied the response time of RDD-1 with PA; it is noted that the colour and fluorimetric change emerged very quickly after addition of PA. It implies that the probe RDD-1 is suitable for quick detection of picric acid. The detection limit (LOD) was found to be 45 nM (3α/S) signifying its feasibility to detect PA even at ultra-low level concentrations. The binding constant of PA-1 with RDD-1 was calculated using Benesi–Hildebrand equation; it is found to be 5.64 × 107 M−1. In order to study the mechanism of photophysical changes during addition of picric acid to RDD-1 density functional theory (DFT) calculations was carried out with the Gaussian 09 program13 with the B3LYP/6-311G basis sets. Geometries of RDD-1 and RDD-1 + PA adduct were initially optimized using above said basis sets. The absorption behaviour and the corresponding transitions are obtained from TDDFT calculations using above said basis sets. (Table S1 in ESI†) The charges on the N atom of dimethyl amino unit in RDD-1 and RDD-1 + PA adduct are found to be −0.628, −0.573 respectively. This significant reduction in the electron density on the N atom upon binding of the picric acid with RDD-1 indicates the significant charge transfer between PA and RDD-1. Also it is found that the charge on both secondary amine N atoms remain unchanged with the addition of PA. Frontier molecular orbital (FMO) analysis further conformed the internal charge transfer (ICT) process occurred after appendage of PA to RDD-1. In RDD-1 HOMO localised on dimethylaminobenzaldimine unit, LUMO spread over on dimethylaminobenzaldimine and partially on rhodamine-6G unit (Fig. 5).
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Fig. 5 Frontier molecular orbitals of RDD-1 and RDD-1 + PA obtained from the DFT calculations using Gaussian 09 program. |
In RDD-1 + PA, HOMO appears over the π moiety of RDD-1 unit whereas LUMO spreads over picric acid. The calculated difference in energy gap of RDD-1 (3.586 eV) decreases significantly to 2.077 eV upon binding with PA. Thus, decrease in energy difference of RDD-1 + PA clearly points out the formation of stronger charge transfer complex of RDD-1 with PA. This also supports the red shift observed in the emission peak. Hence these results clearly point out the intermolecular charge transfer taking place between PA and RDD-1. The involvement of ICT in the emissive state is also confirmed by the observations of solvatochromism in variation of solvent polarity (Fig. S14 in ESI†). In order to demonstrate the practical applicability of RDD-1 for PA detection, a test strip was made by dip coating the solution of RDD-1 on to a filter paper and then subsequently drying in air. The test strips, when immersed into the various concentration of PA in acetonitrile–water mixture, undergo perceptible colour change to red; red fluorescence was noticed upon illumination with hand-held UV lamp (Fig. 6 and 7).
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Fig. 6 Visible (top) colour changes and under UV-light (bottom) of test strips after the addition of picric acid. |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra02931c |
This journal is © The Royal Society of Chemistry 2014 |