Synthesis of a Pyrene-Derived Schiff Base and Its Selective Fluorescent Enhancement by Zinc and Aluminum Ions

An efficient pyrene-Schiff base fluorescent sensor PySb was synthesized and evaluated for its fluorescence response to metal ions. Sensor PySb exhibits an “off-on-type” mode with high selectivity to Zn and Al in ethanol (470 nm) and in dimethyl sulfoxide (458 nm) respectively. The originally non-fluorescent PySb, due to photo-induced electron transfer (PET) from imine moiety, is turned on after binding with the cations. The stoichiometric ratio between PySb and Zn is 1:2; moreover, the limit of detection (LOD) and bonding constant were 2.39 × 10 M and 2 × 10 M respectively, as obtained from titration experiments.


Introduction
Zinc ion has been known as the second most abundant transition metal ion in human body.While most zinc ions are tightly bound in proteins, a small amount of free Zn 2+ ions is presented in various human tissues [1] [2] [3] [4] [5].Some neurologic diseases are found to associate with free Zn 2+ .For instance, the abnormal accumulation of Zn 2+ is found in patients with Alzheimer's disease [6].
Moreover, Zn 2+ plays an important role in the regulation of apoptosis (programmed cell death) [7].Aluminum is the third most abundant element in earth's crust, and it is the most abundant metal on earth.High concentration of aluminum in soils may lead to environmental issues such as acid soils [8] [9].In human body, aluminum is known to associate with the development of Alzhei-P.-F.Hsu, Y. Chen DOI: 10.4236/ijoc.2018.82016208 International Journal of Organic Chemistry mer's disease and Parkinson's disease [10] [11] [12].Therefore, the development of Zn 2+ and Al 3+ sensors for environmental and biological monitoring is critical.
Compare to traditional detection methods such as atomic absorption spectroscopy, electrochemical sensor, and inductively coupled plasma atomic emission spectrometry, fluorescent sensors have become a popular tool for the detection of metal ions due to their simple operating processes [13] [14] [15].
Besides, some of these sensors exhibited responses toward multiple metal ions simultaneously [25] [26].Therefore, it is highly desirable to synthesize fluorescent sensors with specific responses toward multiple metal ions.
In this work, a novel fluorescent sensor PySb was synthesized and characterized.Fluorescent pyrene moiety was selected as fluorophore to enhance sensitivity.2-amino-2-(hydroxymethyl)propane-1,3-diol was chosen as part of binding moiety.By forming imine via reaction with 2-hydroxybenzaldehyde, the three alcoholic -OH along with phenolic -OH should be able to interact with multiple metal ions [27] [28].Accordingly, specific responses toward Zn 2+ and Al 3+ can be expected.

Materials and Measurements
All the reagents and solvents were purchased from commercial sources and were used without further purification.4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid) (HEPES) was dissolved in water (2.5 × 10 −4 M) which was used as buffer solution.Nitrate salts of Na + , K + , Ca 2+ , Cu 2+ , Ni 2+ , Co 2+ , Zn 2+ , Pb 2+ , Fe 3+ , Cr 3+ and Al 3+ were dissolved in the HEPES (aq) buffer to prepare their stock solutions (10 −2 M).PySb was dissolved in ethanol, DMF, or DMSO; the concentration was 2 × 10 −5 M. 1 H NMR spectra with the chemical shifts reported in ppm were recorded on a Bruker AMX-600 MHz spectrometer, using tetramethylsilane (TMS) as an internal standard.Elemental analysis (EA) of carbon, hydrogen, and nitrogen were measured on a Heraus CHN-Rapid elemental analyzer.The UV-Vis absorption spectra were recorded on a Jasco V-550 spectrophotometer using a quartz cuvette (path length = 1 cm).Photoluminescence (PL) spectra were recorded on a fluorescence spectrophotometer (Hitachi F-4500) in mixtures of water with appropriate organic solvents.

Synthesis of Fluorescent Sensor PySb
The new fluorescent sensor PySb was synthesized in two steps as shown in

Photophysical Properties in Ethanol Buffer Solutions
The absorption spectra of PySb with various metal ions in ethanol buffer solutions transitions of pyrene core [29].Upon addition of Na + , K + , Ca 2+ , the absorption of PySb remains nearly unchanged, indicating that PySb does not interact with these ions.On the other hand, the absorption intensities increased with little red or blue shifts (1 -10 nm) when Cu 2+ , Ni 2+ , Co 2+ , Zn 2+ , Pb 2+ , Fe 3+ , Cr 3+ , or Al 3+ were added.This suggests that PySb might have interaction with these ions.
PySb itself exhibits very weak fluorescence in ethanol buffer solution (Figure 3).The fluorescence quenching can be explained by the photo-induced electron-transfer (PET) mechanism [30] [31] as illustrated in

Photophysical Properties in Buffered DMSO Solution
Next, we investigated the photophysical properties of PySb in DMSO solution.
The absorption spectra [Figure 13

1 H NMR Spectra of PySb-Zn 2+ and PySb-Al 3+ Complexes
The formation of PySb-Zn 2+ complex was further investigated by 1   (about 3 hr).Therefore, the small signal changes are probably due to slow formation of PySb-Al 3+ complex.It can be utilized as a fluorescent sensor in a wide range of pH (3 -11).Green emission of PySb (λ em = 515 nm) was observed when pH was higher than 12 due to ICT mechanism.Current results indicate that PySb is a promising fluorescent "turn on" sensor for Zn 2+ and Al 3+ in ethanol and DMSO, respectively.International Journal of Organic Chemistry

Figure 4 .
The fluorescent relaxation of photo-excited electron from pyrene core is forbidden, because the HOMO has been fully filled by obtaining an electron from high-lying HOMO of the terminal 2-iminophenol groups.Upon addition of various metal ions, only Zn 2+ ion caused a significant enhancement of fluorescence intensity (51 times compared to free PySb, λ em = 470 nm).The enhanced sky-blue fluorescence can be clearly observed by naked eyes (Figure5).The result shows that PySb can be applied as an efficient Zn 2+ sensor in ethanol solution.The binding stoichiometry between PySb and Zn 2+ was determined by the Job plot.By plotting the fluorescence intensities versus the molar fraction of Zn 2+ , two regression lines are obtained and they intersect at about 0.66 (Figure6), indicating 1:2 stoichiometric ratio between PySb and Zn 2+ .In other words, each terminal Schiff base moiety in PySb chelates with one Zn 2+ .

Figure 11 demonstratesFigure 12 .
Figure 11 demonstrates pH responses of PySb with and without Zn 2+ from pH = 2.0 to pH = 12.0.PySb did not show significant fluorescence enhancement without Zn 2+ until pH = 12.0.At pH = 12.0, an emission band at 515 nm appears with a fluorescence enhancement (FE) factor of 17.Clearly, the emission arises from the formation of phenolate anion in basic environment as illustrated in Figure 12.(a) Formation of phenolate anion at high pH; (b) Competition between PET and ICT mechanisms.

Figure 15 demonstrates
Figure 15 demonstrates competition effects from other metal ions against Al 3+ sensing.Aside from Cu 2+ which completely quenched the fluorescence as in ethanol solutions, only Fe 3+ partially decreased fluorescence intensity.The result agrees with the observation in ethanol solutions that PySb exhibits higher binding tendency toward Al 3+ .Thus PySb can serve as a highly selective Al 3+ sensor in DMSO solution.

Figure 16 . 1 H 1 H
Figure 16. 1 H NMR of free PySb (bottom), PySb + Zn 2+ (middle), and PySb + Zn 2+ + D 2 O (upper).PySb-Zn 2+ complex might involve an additional hydroxide ion (OH − ).The disappearance of signals of alcoholic (h) and alcoholic protons (a) after addition of D 2 O confirms the presence of these exchangeable protons. 1 H NMR spectra of PySb-Al 3+ complex in DMSO (Figure 17) shows similar results, but the changes in proton signals are much smaller than in DMF.In fact, it took a long time for PySb to reach saturated fluorescence intensity with Al 3+ An efficient fluorescent sensor PySb comprising of pyrene moiety as the fluorophore, benzene ring as the spacer, and 2-(hydroxymethyl)propane-1,3-diol as the ionophore was successfully synthesized and characterized.The PySb itself exhibited weak fluorescence due to PET mechanism; however, the fluorescence P.-F.Hsu, Y. Chen DOI: 10.4236/ijoc.2018.82016221 International Journal of Organic Chemistry