A Naked-Eye Fluoride Ion Recognition Based Vanilin Derivative Chemosensors

Two new receptors S2 and S6 were designed and synthesized based on benzimidazole and azo-benzimidazole synthesized from vanillin for the colorimetric detection of fluoride ion. The presence of nitro group in S2 and azo in S6 makes -OH proton (binding site) acidic and therefore it could deprotonate with addition of basic anion such as fluoride. Hydroxyl (OH) functionality which detects basic fluoride ions via hydrogen-bonding mechanism which is mechanism involved in the color change of receptor R2 and R6, its mean that initial hydrogen bond formation of F − ion with –OH group.


INTRODUCTION
The fluoride anion (F -) is an important trace element in the human body [1]. F is primarily obtained from food and drinking water. Most countries set strict limits on F that is the permitted level of fluorine in foods [2]. It carries both benefits and dangers-a low concentration of F can be effective in treating osteoporosis and the prevention of caries, but excess fluoride will cause dental and skeletal fluorosis, as well as urolithiasis and kidney and gastric disorders [3][4][5][6]. The content of F in natural water is 0.3-0.5 mg/L and can reach 2.0-5.0 mg/L in groundwater with fluorine ore deposits [7]. Over 2.0 mg/L of F in drinking water can cause damage to kidney and liver functions and over 4 mg/L will deform human bones and cause fluorosis [8][9].
In this work, two F ratiometric fluorescent probes were developed for the sensitive detection of F based. The two probes were prepared with nitro-benzimidazole (S2) and azo-nitro-benzimidazole (S6) as the chromophore side and Hidroxyl group as the reactive groups. The two probes could be used for the quantitative detection of F in water. A total of 10 mmol o-phenylendiamine and 4.8 mmol boric acid, each dissolved with 5 mL of warm distilled water at 45 ᵒC then mixed in a beaker glass and stirred for 2 minutes. Then add 5-nitrovaniline 10 mmol which has been dissolved with distilled water temperature of 45 ᵒC, then stirred for 3 minutes. The precipitate formed was recrystallized with ethanol, then the melting point was measured and characterized using FT-IR instruments, mass spectra, 1H-NMR, and 13C-NMR.

METHODOLOGY
The synthesis of S6 compounds was carried out by adopting the method of Kaur et al.
(2015) with some modifications. 4.2 mmol S1 synthesized and 2.8 mmol NaNO2 were each dissolved with 1 mL THF. A total of 0.42 mL 2nitroaniline was dissolved with 6 mL of distilled water and stirred in a beaker glass at 0-5 5C, then dropped with 1.2 mL of HCl 6 M and stirred for 15 minutes, then added a solution of NaNO2 and S1 solution, condition pH by dripping with 2 M NaOH solution to reach pH 8-9. The mixture continues to be stirred for 2 hours, then neutralized the mixture by dripping a 1 M HCl solution. The precipitate formed is then filtered and recrystallized with ethanol. Solvatochromic test is carried out to observe the change in color of the solution towards the difference in the solvent. A total of 0.142 g S2 and 0.19 g S6 were dissolved with 5 mL of DMSO solvent so that the concentration of each sensor in each solvent was 1 × 10-1 M. Then color changes were observed. Furthermore, each solution was diluted to obtain concentrations: 1 × 10-6 M, 1 × 10-7 M, and 1 × 10-8 M. Then absorbance was measured with a UV-vis spectrophotometer.
Saturated anion solution made from NaF salt in distilled water. The ionochromic test is carried out by adding 3 drops of the saturated anion solutions to each solution of the S2 and S6 compounds in the DMSO solvent. The color changes that occur in the solution were observed and then confirmed with a UV-vis spectrophotometer (in the range λ 200-800 nm).

RESULT AND DISCUSSION
S2 dissolves completely in the DMSO solvent, providing optimal absorbance at concentrations of 1 × 10-77 M with a λmax of 470 nm, this is reinforced by the visual solution which shows a brownish yellow color.
S2 solution with a concentration of 1 x 10 -7 M brownish yellow, the addition of Fˉ ions changes the color of the solution to amber color that can be seen with the naked eye ( Figure 1).  (consecutively concentrated from right to left: 0 eq., 1 eqk., 2 eq., 3 eq., 4 eq., 5 eq. , 6 eq.)   Figure 2 shows that with the increasing concentration of ions given, the color of the solution becomes more concentrated. At the addition of 5 × 10ˉ7 M (eq. 1) the color of the solution is still similar to the color S2, starting from the addition of 2 equivalent colors gradually becomes amber, and the amber color becomes more concentrated on the addition of 1.5 × 10ˉ4 M ions (eq. 6).
S6 dissolves completely only in DMSO solvents. The S6 compound in DMSO at a concentration of 1 × 10 M1 M reveals a brownish solution. Measurements on the UV-Vis instrument (concentration 1 × 10ˉ6 M) produce the optimal absorbance achieved at λmax 435 nm, this corresponds to the brownish color that appears.
S6 in DMSO solvent with a concentration of 5 × 10ˉ7 M revealed a brownish clear solution. Tests to determine the response of S6 to ions are carried out by dripping saturated ions (CNˉ, Fˉ, Brˉ, Iˉ, and H2PO4ˉ) 50 μL into the S6 / DMSO solution. The test results are presented in Figure 3.
The sensor response S6 shows that there is a change in color to purple on the addition of Fˉ ions which can be seen directly with the naked eye. Then the S6 response with the F ion ion only shows the color change so that the S6 compound is colory.
Qualitative analysis of the interaction of S6 with Fˉ ions was carried out by titrating a solution of S6 / DMSO 1 × 10ˉ8 M using 50 μL Fˉ ions at several concentrations. Titration results are visually presented in Figure 4.
Measurement of S6-Fˉ interaction strength was performed on the UV-Vis instrument whose measurement results in the form of absorbance spectra are presented in Figure 5. From Figure 5 UV-Vis spectra the titration results show that the absorbance of S6 / DMSO at λ 320 nm with A value of 0.575 and shoulder on λ 350 nm. Absorbance decreased immediately after 5 × 10ˉ7 M (eq. 1) Fˉ ion was added, and at the same time the shoulder formed firmly at λ 575 nm. These results indicate that there is an electron transition π-π * from chromophore.7 The gradual addition of ions to a concentration of 5 × 10ˉ4 M causes the absorbance at λmax to continue to decrease and the absorbance at λshoulder continues to increase, but does not form an isosbestic point..