A Naked-Eye Visible Colorimetric and Ratiometric Chemosensor Based on Schiff Base for Fluoride Anion Detection

A dual-platform colorimetric and ratiometric chemosensor, namely 6-(2hydroxynaphthalen)-N-n-butyl-naphthalimide (HNA), based on Schiff base constructed from N-nbutyl-4-amino-1,8-naphthalimide and 2-hydroxy-1-naphthaldehyde has been designed and fabricated for detecting fluoride anion (F−) in DMSO solution. HNA is a colorimetic and ratiometric probe with superior selectivity and sensitivity. The color changes of HNA from yellow to purple was observed by the naked eyes in the presence of F−. The absorbance in the UV-Vis spectra of HNA was decreasing in 422 nm, while gradually increasing in 583 nm by adding F−. The limit of detection (LOD) of HNA for detecting F− is low to 0.61 μM and the binding model of HNA and F− is in 3:2 stoichiometry. Meanwhile, HNA can be used to detect F− in real samples. Finally, the mechanism of HNA for the detection of F− was investigated. The present work indicated that HNA would be a superior potential chemosensor in monitoring F− selectively and sensitively.

selectivity, simple operation and data analyses, reasonable price and intuitive color changes by naked eyes [31]. Ratiometric sensors take the ratio of the absorption or emission intensities at two wavelengths as signals and usually provide a higher accuracy in quantitative analyses. Ratiometric sensors can eliminate the influence of external environment, detection substrate and photobleaching, probe loading and retention during the experiment [32]. To date, some colorimetric and ratiometric chemosensors for detecting fluoride anion were reported, which are mostly based on the intermolecular interactions, such as desilylation of Si−O/Si−C bonds, B−F complexation and hydrogen bonding interaction [33][34][35][36].
In this work, N-n-butyl-4-amino-1,8-naphthalimide and 2-hydroxy-1-naphthaldehyde were chosen to construct a dual-platform colorimetric and ratiometric chemosensor, namely 6-(2hydroxynaphthalen)-N-n-butyl-naphthalimide (HNA), for the detection of fluoride anion in DMSO solution. The color changes of HNA from yellow to purple in DMSO solution was observed by the naked eyes in the presence of fluoride anion. HNA is a ratiometric chemosensor with the absorbance of the UV-vis spectra decreasing in 422 nm and increasing in 583 nm by adding fluoride anion. The LOD of HNA for detecting fluoride anion is extremely low and HNA can be used to detect fluoride anion in real samples. The mechanism of HNA for detecting fluoride anion has been investigated.

Colorimetric experiments
The UV-Vis spectra of HNA (10 −5 M) towards F − and other anions (Cl − , Br − , I − , NO3 − , HSO4 − , SCN − and ClO4 − ) in TBA salts have been investigated in DMSO solutions. In Figure 1a, the maximum absorption peaks of HNA and HNA with other anions (15 equiv.) were all at 422 nm in the UV-Vis spectra. Once adding 15 equiv. (150 μL) F − , the maximum absorption peak of HNA was red-shifted to 583 nm, where the color of HNA solution turned from yellow to purple observed by naked eyes, while HNA solutions in the presence of other anions showed no obvious changes ( Figure 1b).  In order to further study the ability of HNA for the detection of F − as a selective chemosensor, the competitive experiments were carried out. The maximum absorption of HNA in DMSO solution displayed no obvious decreased after adding 15 equiv. other anions. As listed in Figure 2, the maximum absorption peaks of HNA solutions with F − and with F − and other anions all centered at 583 nm, which implied that HNA would not be interfered by coexisting anions, the existance of F − induced the changes of UV-Vis spectra and HNA would be a selective chemosensor for detecting F − . The colors of HNA solutions with F − and with F − and other anions all changed from yellow to purple. Next, an increasing amount of F − ranging from 0 to 16 equiv. was added to the solution of HNA (10 −5 M) and UV-Vis spectra was measured by titration experiments. The maximum absorption peak at 422 nm decreased gradually, while the main peak at 583 nm appeared and increased ( Figure 3a). The absorbance peak reached to the maximum with the concentration of F − 15 equiv. Obviously, 161 nm red shift in the UV-Vis spectra resulted in the color changes of HNA solution by naked eyes, which is a colorimetric and ratiometric chemosensors. Meanwhile, there existed an excellent linear relationship between the ratiometric values (A583/A422) and the concentration of F − (Figure 3b). The LOD of HNA for detecting F − was low to 0.61 μM, which was calculated based on the equation of LOD = 3σ/S. In the equation, S is meaning of the slope of the calibration and σ is the standard deviation of the response in blank samples without F − [37,38]. Notably, the LOD of HNA for detecting F − is much lower than the limit value in pollutant level for the detection of F − (210 μM) proposed by the US Environmental Protection Agency (EPA) [39]. Some reported chemosensors for the detection of F − have been compared with HNA in Table 1 [40][41][42][43][44].  Finally, Job's plot has been introduced to investigate the stoichiometric proportion of HNA and F − . In the the Job's plot, the maxima peak was at 0.6 in DMSO solution, which was based on the formula of [F − ]/([ F − ] + [HNA]) ( Figure 4). The data implied that the ratio of interaction between HNA and F − was 3:2.

Detecting F − in real samples
HNA sensor has been selected to detect F − in real samples. In this work, the standard addition method for the detection of F − was chosen and the water source was collected from the local water of the Songhua river. The final concentrations of F − in the water samples were 1.0 μM, 5.0 μM, 10.0 μM, 50.0 μM and 100.0 μM, and UV-Vis spectra and linear normalization equation were used to value the data reliability for the detection of F − . The data are listed in Table 2, where the recoveries of HNA for detecting F − were ranging from 96.1% to 104.6% and all RSD values were all within 3.98%. These data implied that HNA as a chemosensor for the detection of F − in real samples is precise and feasible. Average of three repeated measurements of F − . b RSD means relative standard deviation. The possible mechanism of HNA for the detection of F − is the hydrogen bond interaction between HNA and F − at low concentrations, while deprotonation of O−H proton at high concentrations of F − (Scheme 1). Scheme 1. The possible mechanism of HNA for the detection of F − .
To investigate this mechanism of HNA for detecting F − in detail, the orbital energies of HOMO-1, HOMO (highest occupied molecular orbital), LUMO (lowest unoccupied molecular orbital) and LUMO+1 of HNA, HNA+F − and HNA − have been calculated by the method of DFT (density functional theory) at the B3LYP/6-31G* level ( Figure 5). Obviously, the △Egap values of HNA, HNA+F − and HNA − were 0.325 eV, 0.090 eV and 0.085 eV, respectively, which decreased gradually. The electron affinities of HNA+F − and HNA − were relatively higher than HNA, resulting in the transfer of electrons in HNA+F − and HNA − more easily than HNA. Meanwhile, the △Egap values of HNA+F − and HNA − were smaller than HNA, and it implied that the interaction between HNA and F − was hydrogen bond, while deprotonation at high concentration of F − [45][46][47]. Furthermore, the electrons transitions of HNA+F − and HNA − in HOMO-1→LUMO and HOMO→LUMO+1 may produce the same results.

Materials and physical measurement
All the reactants and solvents used in this work were commercially available without purification. Anions of F − , Cl − , Br − , I − , NO3 − , HSO4 − , SCN − and ClO4 − in this work were obtained from tetrabutylammonium (TBA) salts and purchased from Sigma-Aldrich. Dimethyl sulfoxide (DMSO) was purchased from Aladdin. High resolution mass spectrometer (HRMS) was measured on Agilent 6224. The 1 H NMR spectra were tested on a Bruker AVANVE 400 MHz nuclear magnetic resonance spectrometer (Bruker, Germany) utilizing CDCl3 and DMSO-d6 as the solvents, where TMS is the internal standard. The fusion points were recorded on Shanghai Inesa melting point apparatus (WRS-3) without correction. Finally, UV-Vis spectra were measured with the range of wavelengths from 200 nm to 800 nm on a Shimadzu UV-2700 spectrophotometer.

Computational details
All the calculations were conducted with the Gaussian 09W software package [51]. The geometries were optimized by the B3LYP functional [52,53]. The 6-31G* basis set was utilized for all atoms. All geometry optimizations were carried out without symmetry constraints in conjunction with the PCM continuum solvation model [54], which used the solvent of water.

Conclusions
In summary, a colorimetric and ratiometric chemosensor HNA based on Schiff base was designed and synthesized. The color changes of HNA could be observed by the naked eye in the case of detecting F − , while other anions showed no color changes. HNA exhibited superior sensitivity and selectivity for the detection of F − with low LOD of 0.61 μM. The recognition mechanism of HNA for the detection of F − was further verified by the method of theoretical calculation. The interaction between HNA and F − was hydrogen bond, while deprotonation at high concentration of F − . Moreover, HNA could be used for detecting F − in real samples conveniently and precisely.