INTRODUCTION

The synthesis and modification of mesoporous silica have recently received much attention, due to its large surface area and pore volume, tunable and narrowly distributed pore diameter, controllable surface functionalization, chemical inertness, and biocompatibility, all of which have made it ideal inorganic matrix for use in adsorption, ion exchange, catalysis and sensing fields [1-3].

Iron is one of the most important elements in the biological systems, playing a significant role in the oxygen transport, storage and in the electron transport [4]. Regarding its industrial use, iron and its compounds have numerous important applications. However, despite the urgent need for iron-selective sensors for the potentiometric monitoring of Fe⁺³ ions in chemical, biological, industrial and environmental samples, there have been only limited reports of such solid-state electrochemiluminescence sensor in the literature [5, 6]. The need for determination of iron in clinical, medicinal, environmental and industrial samples has, the one hand, led to a number of methods for its measurement [7]. Previously, we reported the fluorene functionalized mesoporous silica based on mono, di and tri ammonium functionalized SBA-15 and studied its flourescence behavior in presence H⁺ and Fe³⁺ [8, 9]. Also reported the synthesis of the fluorene functionalized mesoporous silica based on bis (2-aminoethyl)-2-(9-fluorenyl) malonamide to identify iron ion [10].

Herein, fabrication of Fe³⁺-optical sensor based on SBA-15 and Diethyl 2 - (9 -fluorenyl) malonate that functions as a flourophore in aqueous solution. In view of high chemical stability and fluorescent emission of fluorenyl, first we linked it as fluorophore to the dioxotetramine receptor then, to the 3-aminopropyltriethoxysilane and finally to the inner surface of mesoporous support SBA-15. In the presence of Fe³⁺, this sensor shows highly selective and sensitive recognition toward Fe³⁺ over a wide range of tested cations including Mg²⁺, Cr³⁺, Co²⁺, Ni²⁺, Cu²⁺, Hg²⁺ and Zn²⁺ without fluorescence intensity change. Compared with other reported fluorescent chemosensors for Fe³⁺ ions, this system showed the highest selectivity toward Fe³⁺ ions in aqueous solution.

MATERIALS AND METHODSS

Materials

EO₂₀PO₇₀EO₂₀ (P123, MW= 5800) was obtained from Aldrich. Fluorene, bromine, carbon tetrachloride, tetraethyl orthosilicate (TEOS), 3-aminopropyltriethoxysilane, Iron (ΙΙ) sulfate, sodium carbonate, diethyl malonate and hydrochloric acid were obtained from Merck. . Iron (ΙΙΙ) nitrate, magnesium nitrate, cupper (ΙΙ) nitrate, cobalt (ΙΙ) nitrate, nickel (ΙΙ) nitrate, chromium (ΙΙΙ) nitrate, mercury (ΙΙ) nitrate and zinc nitrate were of analytical grade. Toluene and DMF were dried according to the standard purification methods [11].

Synthesis of SBA-15

SBA-15 material was prepared according to literature procedures [8, 10].

Synthesis of diethyl 2 - (9 -fluorenyl) malonate (Fm)

9-bromofluorene was prepared by literature methods [8, 12].The diethyl 2 -(9 -fluorenyl) malonate material was synthesized in accordance with the procedure reported by Luo et al [13]. To a solution of diethyl malonate (4.28 g, 0.027 mol) in anhydrous DMF (5 ml) were added Na₂CO₃ (2.83 g, 0.027 mol) and a solution of 9-bromofluorene (6.56 g, 0.027 mol) in DMF (20 ml) are added. The mixture was stirred at 70 °C for 24 h and subsequently poured into 250 ml ice water. The formed suspension was extracted three times with 25 ml portions of diethyl ether. The organic layer was dried over Na₂SO₄ and the solvent was removed by rotary evaporation to give a yellow solid (Fig. 1). After recrystallization from ethanol, yellow needle crystals were obtained; m.p. 69–72 °C, ν_max (cm^-1) 2977 (ν_CH), 1727 (ν_CO), 1447 and 1367 (ν_CH), 735 (ν_C=C).

Sylilation of diethyl 2 - (9 -fluorenyl)malonate (Fm-Si)

The Fm-Si was synthesized based on ref [13]. The mixture of the diethyl 2 -(9 -fluorenyl) malonate (6.28 g, 0.001 mol) and 3-aminopropyltriethoxysilane (5 ml) was stirred at 35 °C for 72 h. Excess 3-aminopropyltriethoxysilane was distilled off under reduced pressure. After cooling, the yellow residue was treated and washed with diethyl ether ether (Scheme 1). ν _max (cm^-1) 2971 (ν_CH),1740 (ν_CO), 1446 (ν_NH), 1049 (ν_SiO) and 786 (ν_C=C).

Synthesis of fluorene functionalized SBA-15 (SBA-Fm-Si)

0.1 g of SBA-15 was placed in a vacuum for 30 min, then 20 mL of dry toluene was added and stirred for 15 min. afterwards the Fm-Si (0.1 g, 0.285 mmol) was added to the resulting mixture under an inert atmosphere and refluxed for 24 h. Next, it was cooled at room temperature and the toluene was filtered and dried.

Instruments and spectroscopic measurements

Nitrogen physisorption isotherms were obtained on a BELSORP mini-II at liquid nitrogen temperature (77K). Surface area was measured using the Brunauer-Emmett-Teller (BET) method, pore size distributions were calculated from the nitrogen isotherms by Barrett-Joyner-Halenda (BJH) method. Small and wide angle x-ray scattering (SAXS) patterns were recorded with a model Hecus S3-MICROpix SAXS diffractometer with a one-dimensional PSD detector using Cu Kα radation (50KV, 1mA) at wave length 1.542 Å.FT-IR spectra were recorded a 4000-600 cm^-1 region on Equinox 55 spectrometer. Thermogravimetric analysis (TGA) was performed on a TA Q50 instrument. The scans were performed between 20 °C and 900 °C at 10 °C/min. Emission spectra were recorded on Perkin-Elmer LS50 model luminescence spectrometer. Fluorescence measurements were done in a 1 cm quartz cuvette containing a magnetic-stirred solution of compounds in aqueous solvent (0.1g L^-1). Calcinations were carried out in a box furnace Gofa in the presence of flowing air. Melting points were determined by the capillary method on electro thermal 9100 apparatus.

RESULTS AND DISCUSSION

Small and wide angle X-ray diffraction

Fig. 2 shows X-ray diffraction patterns of SBA-15 and SBA-Fm-Si. Both samples have a single intense reflection around 2θ = 0.87°is characteristic of the long-range periodic in SBA-15 materials ^[14]. Two additional peaks around 2θ = 2° relate to the higher ordering (110) and (200) reflections, which are associated with a two-dimensional hexagonal (p6mm) structure [14]. However, the intensity of (100) reflection in SBA-Fm-Si is decreased due to the difference in the scattering contrast of the pores and the walls, and to the irregular coverage of organic groups on the nanochannels. The presence of those three reflections confirms that the hexagonal structure of the SBA-15 is preserved without collapsing during the modification of SBA-15 the surface. The larger wall thickness of SBA-Fm-Si is consistent with grafting the organic groups onto the pore walls. (Table1).

N₂ adsorption-desorption

Fig. 3 shows the N₂ adsorption-desorption isotherms of SBA-15 and SBA-Fm-Si. Both materials exhibit a typical irreversible type IV nitrogen adsorption isotherm with an H1 hysteresis loop as defined by IUPAC ^[14]. There is a shift of the hysteresis position toward low relative pressures and a slight decreasing trend in overall N₂ adsorption volume as the loading of Fm-Si groups. The pore size distribution was calculated from BJH method based on the desorption branch of the N₂ adsorption isotherm. As demonstrated in the inset of Fig. 3, a BJH plot of SBA- Fm-Si shows a narrow and uniform pore size distribution which is comparable to the original SBA-15, indicating that the integrity of the original structure of the SBA-15 was retained. In comparison with SBA-15 decreasment in the pore diameter value by about 1.4 nm is observed for SBA-Fm-Si. The textural parameters including specific surface areas (BET method), pore diameters (BJH method) and total pore volumes are given in Table 1. The decreasing of all three parameters in SBA-Fm-Si is due to the functionalization inside the channels of SBA-15.

Fig. 4 shows FT-IR spectra of SBA-15 and SBA-Fm-Si. The band at 3434 cm^-1 in SBA-15 spectrum can be assigned to the stretching vibration of OH groups. The bands at 800 and 1086 cm^-¹ are attributed to Si-O-Si and Si-O stretching vibrations, respectively [15]. After grafting Fm-Si on the surface of SBA-15 in SBA-Fm-Si, the intensity of the band at 3434 cm^-¹ is decreased. The band at 2930 cm^-¹ is assigned to C-H stretching vibrations of the methylene groups. Moreover SBA-Fm-Si spectrum show the band at 1670 cm^-1 which is the typical stretching frequency of the carbonyl in amid and the strong peak of 1450 cm^-1 was observed which due to the C=C ring skeletal vibrations. These results indicated that the Fm-Si group was attached on the surface of SBA-15.

Fig. 5 gives the TGA profile of SBA-Fm-Si. The weight loss at around 100 °C corresponded to desorption of physisorbed water [16]. The weight loss 16.3 % at 180-600 °C is mainly due to the decomposition of Fm-Si groups in SBA-Fm-Si.

The fluorescence emission spectra of the suspension SBA-Fm-Si (200 μg/2.0 mL) in water exhibited two peaks at 310 nm and 410 nm (λ_ex = 260 nm) due to the grafted fluorene groups on the mesoporous silica (Fig. 7). The second emission could be observed between 360 nm to 410 nm for fluorene that depends on the media [17]. This emission is attributed to a novel kind of twisted intermolecular charge transfer (TICT) conformer within the life time of the lowest excited singlet state [18]. With the addition of Fe³⁺, the emission peak intensity is significantly decreased. However no obvious changes could be observed upon the addition of Mg²⁺, Cr³⁺, Co²⁺, Ni²⁺, Cu²⁺, Hg²⁺ and Zn²⁺ (Fig. 6).

These observations indicated that SBA-Fm-Si had a high sensitivity and excellent selectivity for Fe³⁺ in water. The results offluorimetric titration indicate that fluorescence of fluorenyl was quenched by transfer of electron from metal center to photoexcited state of fluorenyl during coordination of Fe³⁺. As shown in Fig. 7, a decrease of fluorescence intensity could be remarkably observed with an increasing Fe³⁺ concentration.

This high selectivity can be interpreted in terms of different charge density of metal ions. It is well known that the charge density (ρ) of a metal ion is defined as the amount of electric charge/unit volume. It is one of the most important characterizations of the relative electrophilicity of a metal ion. The charge densities of metal ions were calculated according to the Eq. (1):

 (1)

Where q is the formal charge and r denotes the Shannon ionic radius (Table 2) [19]. The calculated charge density of the cations are given in the table 2 and increased in the following order: Hg²⁺ (0.45) < Co²⁺ (1.15) < Zn²⁺ (1.18) < Cu²⁺ (1.22) < Mg²⁺ (1.28) < Ni²⁺(1.45) < Cr³⁺ (3.08) < Fe³⁺ (6.09).

The results indicate that the charge density of Fe³⁺ ion is the biggest among all of the competition ions. Therefore, the electrophilic ability of Fe³⁺ ion is the strongest among cations that can be used to interpret the capability of forming a coordination complex with an electron-rich ligand. Thus, the strong coordinate interaction between Fe³⁺ ions with the lone pair of the amine groups in SBA-Fm-Si significantly quench the fluorescence of fluorene because of the strong electrophilicity ability when it mixes with the other competition ions in the analytic solutions.

Fig. 8 shows the plot of (I₀ - I)^-1 vs. [Fe³⁺]^-1, where I₀ and I refer to the emission fluorescence intensity at 310 nm for SBA-Fm-Si in the absence and presence of Fe³⁺ ions, respectively.

Generally, the mechanism of fluorescence quenching is divided into three types: dynamic quenching; static quenching; energy transfer [20]. Each of them is depicted through its special equation. A good linearity should be developed in the plot of (I₀ - I)^-1 vs. [Fe³⁺]^-1 for the static quenching mechanism according to the following Line weaver-Burk equation (2) [21]:

 (2)

As it seen in Fig. 8, a good linearity between (I₀ - I)^-1 and the reciprocal of the concentration of Fe³⁺ from 0.16 to 0.83 mM (linearly dependent coefficient R² = 0.99) is constructed. According to the above equation, the dissociation constant K_D can be obtained from the slope and intercept of the line and the corresponding association constant K (1/K_D) is 3.2×١٠⁴. Moreover the detection limit of SBA-Fm-Si for monitoring of Fe³⁺ are calculated according to Eq. (3), where DL is the minimal analytical signal that can be detected, S_D is the standard deviation of blank and m is the plot slope of fluorescence intensity vs. [Fe³⁺].

 (3)

Therefore detection limit of SBA-Fm-Si was calculated to be 1.99×١٠^-5.

Based on consistent of fluorescence titration with the Line weaver-Burk equation, we could tentatively assign this quenching process as a static quenching mechanism that involves the interaction between the fluorescence molecule and the metal ions to form a nonfluorescent complex. This interaction often takes place in the ground state of the fluorescence molecule, and the quenching efficiency is governed by the formation constants and the concentration of metal ions [22].

CONCLUSION

We have introduced a selective and sensitive optical sensor to detect aqueous Fe³⁺ based on fluorene functionalized mesoporous silica. The materials were characterized using different techniques. Small and wide angle X-ray analysis showed that the mesoporous structure was preserved during process. N₂ adsorption-desorption isotherms display opening pores. Surface area and pore size were decreased by attaching the Fm-Si to the pore surface. FT-IR spectra demonstrated the incorporation of Fm-Si groups on the surface of SBA-15. Thermogravimetric analysis also indicates the successful immobilization of organic groups on to the surface of mesoporous silica. The sensing ability of SBA-Fm-Si was studied by fluorescence spectroscopy in presence of Fe³⁺, Mg²⁺, Cr³⁺, Co²⁺, Ni²⁺, Cu²⁺, Hg²⁺ and Zn²⁺. In response to Fe³⁺, the system provided remarkable fluorescence intensity change. Moreover, compared with the reported sensors for Fe³⁺ ions, this is the second chemosensor based on diethyl 2 -(9 -fluorenyl)malonate functionalized SBA-15 that can selectively detect Fe³⁺ in an aqueous environment. A good linearity between the fluorescence intensity of SBA-Fm-Si and the concentration of Fe³⁺ ion was observed, which enables this martial as a fluorescence chemosensor for detecting the Fe³⁺ ion with a suitable detection limit of 1.99×١٠^-5. We believe that the sensor can be promoted for a lot of sensible applications in chemical, environmental and biological systems.

ACKNOWLEDGMENTS

The authors wish to thank the University of Tehran for financially supporting this work.

CONFLICT OF INTEREST

The authors declare that there are no conflicts of interest regarding the publication of this manuscript.