International Journal of Molecular Sciences Article A Benzothiazole-Based Fluorescent Probe for Ratiometric Detection of Al3+ and Its Application in Water Samples and Cell Imaging Zhen-Nan Tian, Ding-Qi Wu, Xue-Jiao Sun, Ting-Ting Liu and Zhi-Yong Xing * Department of Applied Chemistry, College of Arts and Sciences, Northeast Agricultural University, Harbin 150030, China; [email protected] (Z.-N.T.); [email protected] (D.-Q.W.); [email protected] (X.-J.S.); [email protected] (T.-T.L.) * Correspondence: [email protected] Received: 29 October 2019; Accepted: 26 November 2019; Published: 28 November 2019 Abstract: An easily prepared benzothiazole-based probe (BHM) was prepared and characterized by general spectra, including 1H NMR, 13C NMR, HRMS, and single-crystal X-ray diffraction. Based on the synergistic mechanism of the inhabitation of intramolecular charge transfer (ICT), the BHM 3+ displayed high selectivity and sensitivity for Al in DMF/H2O(v/v, 1/1) through an obvious blue-shift in the fluorescent spectrum and significant color change detected by the naked eye, respectively. The binding ratio of BHM with Al3+ was 1:1, as determined by the Job plot, and the binding details were investigated using FT-IR, 1H NMR titration, and ESI-MS analysis. Furthermore, the BHM was successfully applied in the detection of Al3+ in the Songhua River and on a test stripe. Fluorescence imaging experiments confirmed that the BHM could be used to monitor Al3+ in human stromal cells (HSC). Keywords: benzothiazole; ratiometric; fluorescent probe; Al3+ 1. Introduction Aluminum, as one of the most abundant metals in the earth, is widely used in daily life, including in kitchen utensils, pharmaceutical packaging, and food additives [1–4]. These activities will inevitably induce the accumulation of Al3+, which is the most ionic style of aluminum in existence in environmental water and the human body. However, as a non-necessary element for humans, excess accumulation of Al3+ in the human body might be associated with diseases, including Alzheimer’s disease, Parkinson’s disease, amyotrophic sclerosis, and encephalopathy [5–10]. According to the regulations of the World Health Organization (WHO), the maximum concentration of Al3+ is 7.4 µM in drinking water. Hence, it is necessary to construct a simple and effective method with the function of qualitative and quantitative analysis in Al3+ detection. The development of fluorescent probes for detection and special analysis has attracted many researchers, and numerous excellent probes for Al3+ have been reported [11–27]. Among them, the ratiometric probe is one of the most popular types of sensors due to the inherent advantage that it can eliminate the interference of the external environment through self-calibration of its response signals either in emission or absorbance intensities at two different wavelengths. Some ratiometric Al3+ probes were developed using different fluorophores, including rhodamine–naphthalene [17], rhodamine-benzothiazole [18], 40-methyl-3-hydroxyflavone [19], coumarin-quinoline [20], rhodamine- chromone [21], carbazole-rhodamine [22], imidazoquinazoline [23], benzothiazole [24], benzimidazole [25], naphthalimide [26], and quinoline–coumarin [27]. However, only a few of these were benzothiazole- based fluorescent probes for the ratiometric detection of Al3+ [24]. Moreover, benzothiazole, an excellent Int. J. Mol. Sci. 2019, 20, 5993; doi:10.3390/ijms20235993 www.mdpi.com/journal/ijms Int. J. Mol. Sci. 2019, 20, x FOR PEER REVIEW 2 of 11 stability and easy structural modification, had been used in the construction of chemosensors for the Int. J. Mol. Sci. 2019, 20, 5993 2 of 12 Int.detection J. Mol. Sci. of 2019 various, 20, x FORions PEER [28– REVIEW34]. Moreover, this type of structure, in which the hydroxyl group2 ofacts 11 as an electron donating unit in the para-position of the benzothiazolyl group used as the electron stabilitywithdrawing and easy group, structural was facile modif forication the ICT, had-based been mechanism used in the byconstruction increasing of the chemosensors push-pull electron for the fluorophore due to its distinguished merits, such as excellent photo stability and easy structural detectioneffect [35]. of various Given ions the [28 –34above]. Moreover, statements, this type aof structurenovel ,eas in ilywhich prepared the hydroxyl probe group BHM acts modification, had been used in the construction of chemosensors for the detection of various ions [28–34]. as(5- (anbenzo[d]thiazol electron donat-2ing-yl) -unit2-hydroxy in the -para3-methoxybenzaldehyde-position of the benzothiazolyl) was synthesized group used, and as theits electron stereo Moreover, this type of structure, in which the hydroxyl group acts as an electron donating unit in the withdrawingstructure was group,confirmed was by facile single for crystal the ICT X-ray-based diffraction mechanism (Scheme by increasing 1). The BHM the showedpush-pull an electronobvious para-position of the benzothiazolyl group used as the electron withdrawing group, was facile for the effectselectivity [35]. and Givensensitivity the to Alabove3+ with statements, a significant ablue novel-shift botheas ilyin theprepared emission probespectrum BHM and ICT-based mechanism by increasing the push-pull electron effect [35]. Given the above statements, (absorbance5-(benzo[d]thiazol spectrum.-2-yl Its)-2 application-hydroxy-3 -methoxybenzaldehydein real water samples was) was investigated. synthesized Moreover,, and its the stereoBHM a novel easily prepared probe BHM (5-(benzo[d]thiazol-2-yl)-2-hydroxy-3-methoxybenzaldehyde) structurewas successful was confirmed in imaging by the single human crystal stromal X-ray cell diffraction line (HSC (Scheme). 1). The BHM showed an obvious was synthesized, and its stereo structure was confirmed by single crystal X-ray diffraction (Scheme1). selectivity and sensitivity to Al3+ with a significant blue-shift both in the emission spectrum and The BHM showed an obvious selectivity and sensitivity to Al3+ with a significant blue-shift both in the absorbance2. Results and spectrum. Discussion Its application in real water samples was investigated. Moreover, the BHM emission spectrum and absorbance spectrum. Its application in real water samples was investigated. was successful in imaging the human stromal cell line (HSC). Moreover,2.1. Synthesis the of BHM BHM was successful in imaging the human stromal cell line (HSC). 2. Results and Discussion 2.1. Synthesis of BHM Scheme 1. Synthetic route of the benzothiazole-based probe (BHM). Scheme 1 Synthetic route of the benzothiazole-based probe (BHM). 2. ResultsThe BHM and Discussion was facile synthesized through a Duff reaction using the starting material of 4-(benzo[d]thiazol-2-yl)-2-methoxyphenol (3), which was synthesized via a condensation reaction 2.1. Synthesis of BHM between 2-aminobenzenethiolScheme 1 Synthetic and 4 -routehydroxy of the-3 -benzothiazolemethoxybenzaldehyde-based probe ([36].BHM The). single crystal of BHMThe was BHM obtained was, and facile its molecular synthesized structure through is depicteda DuDuffff reactionreaction in Figure usingusing 1. The the selective starting crystal material data are of 44-(benzo[d]thiazol-2-yl)-2-methoxyphenoldisplayed-(benzo[d]thiazol in Table- S12-yl and)-2- methoxyphenolTable S2. ( (3),3), which which was was synthesized via via a condensation reaction between 2-aminobenzenethiol2-aminobenzenethiol and 4-hydroxy-3-methoxybenzaldehyde4-hydroxy-3-methoxybenzaldehyde [[36].36]. The The single crystal of BHM was obtained,obtained, andand itsits molecularmolecular structurestructure isis depicteddepicted inin FigureFigure1 1.. The The selective selective crystal crystal data data areare displayed in TablesTable S1 S1 and and Table S2. S2. Figure 1. X-ray crystal structure of BHM (heteroatoms N, O and S were labeled in different color). 2.2. Spectra Performance Investigation of the BHM to Metal Ions TFigureheFigure selectivity 1. XX-ray-ray ofcrystal crystal BHM structure structure (10 μM of of) toBHM BHM different (h (heteroatomseteroatom metals ionsN, O (and Na +S , Kwere +, Mg labeled 2+, Ca 2+in , didifferentAgfferent+, F e 2+ color).color), Co2+. , Ba2+, Mn2+, Cu2+, Zn2+, Cd2+, Hg2+, Ni2+, Pb2+, Cr3+, Fe3+, and Al3+) was firstly determined using UV-Vis 2.2. Spectr Spectraa Performance Performance Investigation of the BHM to Metal Ions spectrum in a solution of DMF:H2O (1:1, v/v) (Figure 2). The UV-Vis absorbance of BHM showed + + 2+ 2+ + 2+ 2+ almostTThehe no selectivity change after of BHM the addition( (1010 μMµM)) to toof different dithffe erenttested metal metal metal ions ions ions ( (NaNa, except+, ,KK+, Mg ,for Mg2+ Al, Ca,3+ Ca 2+in, Ag DMF:H,+ Ag, F e,2+2 FO, Co e (1:1,2+,, Co Ba v/v2+),, 2+ 2+ 2+ 2+ 2+ 2+ 2+ 2+ 3+ 3+ 3+ MnBa 2+,, MnCu2+, ,Zn Cu2+, ,Cd Zn2+, Hg, Cd2+, Ni, Hg2+, Pb, Ni2+, Cr,3+ Pb, Fe,3+ Cr, and, FeAl3+,) andwas Al firstly) was determined firstly determined using UV using-Vis spectrumUV-Vis spectrum in a solution in asolution of DMF:H of2 DMF:HO (1:1, 2v/vO) (1:1, (Figurev/v) 2 (Figure). The 2UV).- TheVis UV-Visabsorbance absorbance of BHM of showed BHM almost no change after the addition of the tested metal ions, except for Al3+ in DMF:H2O (1:1, v/v) Int. J. Mol. Sci. 2019, 20, 5993 3 of 12 showed almost no change after the addition of the tested metal ions, except for Al3+ in DMF:H O Int. J. Mol. Sci. 2019, 20, x FOR PEER REVIEW 3 of 11 2 (1:1, v/v) (Figure2a). As shown in Figure2a, upon the addition of Al 3+, an obvious blue-shift from an absorbance(Figure peak2a). As at 359shown to 321in nmFigure was 2a, observed upon the compared addition toof thatAl3+, ofan the obvious BHM blue itself,-shift which from indicated an the existenceabsorbance of apeak strong at 359 interaction to 321 nm betweenwas observed the BHM compared and Alto 3that+. Furthermore, of the BHM itself, the which absorbance indicated titration resultthe (Figure existence2b) showedof a strong the absorbanceinteraction between centered the at 359BHM nm and was Al gradually3+.