SBA-Pr-Is-TAP Functionalized Nanostructured Silica as a Highly Selective Fluorescent Chemosensor for Fe3+ and Cr2O72− Ions in Aqueous Media

SBA-Pr-Is-TAP was synthesized via functionalization of SBA-15. The synthesized hybrid nanomaterial was characterized by various techniques including FT-IR, TGA, XRD, SEM, and BET. SBA-Pr-Is-TAP could precisely bind Fe3+ and Cr2O72− ions among a range of different species in aqueous media, consequently acting as a nanoporous chemosensor of Fe3+ and Cr2O72− ions. An excellent linear relation was observed between the nanoporous chemosensor and ion concentrations, with acceptable detection limits of 2.43 × 10−6 M and 3.96 × 10−7 M for Fe3+ and Cr2O72− ions respectively.

Since isatin and its derivatives have excellent properties including biological and pharmacological activities, these have been used in various organic syntheses [19][20][21]. Functionalized nanoporous materials by Fe 3+ ion can play a vital role in important processes including the transformation of an electron in DNA and RNA as well as biological metabolism [22]. The lack or excess of iron has been reported to lead to a number of diseases such as skin problems, iron shortage anemia (IDA), low immunity, Alzheimer's, Huntington's, and Parkinson's [23]. The increase of iron could disturb biological systems

Results and Discussion
The overall process for the preparation of the SBA-Pr-Is-TAP is illustrated in Scheme 1. Firstly, SBA-15 was functionalized with (3-chlorpropyl) triethoxysilane to provide SBA-Pr-Cl, which was modified by isatin to give SBA-Pr-Is. In the final step, SBA-Pr-Is were reacted with 2,4,6-triaminopyrimidine to provide SBA-Pr-Is-TAP, fully characterized by different analysis methods including IR, TGA, XRD, etc. Full information on characterization equipment and experiments is provided in the experimental section.
Three amine groups on 2,4,6-triaminopyrimidine can potentially react with carbonyl groups for the formation of a Schiff Base. NH2 groups in positions of 4 and 6 are chemically equivalent, therefore two types of amine groups present for Schiff base reaction (two amine groups at positions of 4 and 6, and the other at position 2). On the other hand, the basicity of the amino group at position 2 is lower than the others, suggesting that the surface adduct should be formed via reaction at the 4 or 6 positions. FT-IR spectroscopy shows the organic moieties covalently bonded on the SBA-15 surface. FT-IR spectra for SBA-15, SBA-Pr-Cl, SBA-Pr-Is, and SBA-Pr-Is-TAP are depicted in Figure 1. The bands at 800 and 1100 cm −1 , respectively, are related to Si-O-Si bonds, while the band at 960 is related to Si-OH. Then, the band around 3456 cm − ¹ could be attributed to O-H stretching of Si-OH functional groups. The peak around 1650 cm − ¹ is associated with C=O stretching vibration and the bending vibrations of water molecules (in spectrum c). The presence of a new band in spectra b and c in at 2881-2990 cm − ¹ range is due to the stretching vibrations of methylene in propyl groups. In c, the new band at 1479 and 1754 cm⁻¹ corresponded to C=C and C=O stretching vibrations. The new band at 1706 is related Scheme 1. Schematic preparation of SBA-Pr-Is-TAP.
Three amine groups on 2,4,6-triaminopyrimidine can potentially react with carbonyl groups for the formation of a Schiff Base. NH 2 groups in positions of 4 and 6 are chemically equivalent, therefore two types of amine groups present for Schiff base reaction (two amine groups at positions of 4 and 6, and the other at position 2). On the other hand, the basicity of the amino group at position 2 is lower than the others, suggesting that the surface adduct should be formed via reaction at the 4 or 6 positions.
FT-IR spectroscopy shows the organic moieties covalently bonded on the SBA-15 surface. FT-IR spectra for SBA-15, SBA-Pr-Cl, SBA-Pr-Is, and SBA-Pr-Is-TAP are depicted in Figure 1. The bands at 800 and 1100 cm −1 , respectively, are related to Si-O-Si bonds, while the band at 960 is related to Si-OH. Then, the band around 3456 cm −1 could be attributed to O-H stretching of Si-OH functional groups. The peak around 1650 cm −1 is associated with C=O stretching vibration and the bending vibrations of water molecules (in spectrum c). The presence of a new band in spectra b and c in at 2881-2990 cm −1 range is due to the stretching vibrations of methylene in propyl groups. In c, the new band at 1479 and 1754 cm −1 corresponded to C=C and C=O stretching vibrations. The new band at 1706 is related to C=N groups. In spectra d, the new band at 3400 cm −1 is related to NH 2 groups of triaminopyrimidine. These new bands proved that aliphatic compounds, isatin, and triaminopyrimidine were successfully immobilized on the surface of SBA-15. to C=N groups. In spectra d, the new band at 3400 cm − ¹ is related to NH₂ groups of triaminopyrimidine. These new bands proved that aliphatic compounds, isatin, and triaminopyrimidine were successfully immobilized on the surface of SBA-15. X-ray powder diffraction (XRD) of SBA-15 and SBA-Pr-Is-TAP is shown in Figure 2. According to the obtained results, a strong reflection was observed at around 2θ = 1° corresponding to (100) plane, while the other two intense reflections at ca. 2θ = 1.8° and 2.0° could be assigned to (110) and (200) planes. These reflections demonstrate that the functionalized products maintained highly ordered mesoporous hexagonal structures. Comparing the XRD of SBA-15 with SBA-Pr-Is and SBA-Pr-Is-TAP, the same diffraction lines could be visualized in all three materials (with different intensities). These results confirmed the successful incorporation of the organic moieties onto SBA-15.
SEM and TEM images also proved the nano-ordered arrays of SBA-15 and SBA-Pr-Is-TAP hexagonal nanoporous channels, as shown in Figures 3 and 4. The prepared materials' physical properties and porosity were investigated by nitrogen adsorption-desorption isotherms ( Figure 5). These compounds display type IV isotherms with H1-type hysteresis loops, which proved the pore walls of SBA-Pr-Is and SBA-Pr-Is-TAP are still open. After the attachment of organic compounds on SBA-15, certain changes in physical properties became evident including a reduction in surface area (SBET), average pore diameter (dp), and pore volume (Vp) as shown in Table 1. The reasons for the reduced surface area, pore diameter, and pore volume relate to the deterioration of the textural properties of SBA-15 upon the incorporation of the organic moieties (pore diameter and volume) due to partial pore blocking of such organic moieties and surface area due to the partial covering of the surface with the anchored organic moieties. X-ray powder diffraction (XRD) of SBA-15 and SBA-Pr-Is-TAP is shown in Figure 2. According to the obtained results, a strong reflection was observed at around 2θ = 1 • corresponding to (100) plane, while the other two intense reflections at ca. 2θ = 1.8 • and 2.0 • could be assigned to (110) and (200) planes. These reflections demonstrate that the functionalized products maintained highly ordered mesoporous hexagonal structures. Comparing the XRD of SBA-15 with SBA-Pr-Is and SBA-Pr-Is-TAP, the same diffraction lines could be visualized in all three materials (with different intensities). These results confirmed the successful incorporation of the organic moieties onto SBA-15.
SEM and TEM images also proved the nano-ordered arrays of SBA-15 and SBA-Pr-Is-TAP hexagonal nanoporous channels, as shown in Figures 3 and 4. The prepared materials' physical properties and porosity were investigated by nitrogen adsorption-desorption isotherms ( Figure 5). These compounds display type IV isotherms with H1-type hysteresis loops, which proved the pore walls of SBA-Pr-Is and SBA-Pr-Is-TAP are still open. After the attachment of organic compounds on SBA-15, certain changes in physical properties became evident including a reduction in surface area (S BET ), average pore diameter (d p ), and pore volume (V p ) as shown in Table 1. The reasons for the reduced surface area, pore diameter, and pore volume relate to the deterioration of the textural properties of SBA-15 upon the incorporation of the organic moieties (pore diameter and volume) due to partial pore blocking of such organic moieties and surface area due to the partial covering of the surface with the anchored organic moieties.            Thermogravimetry studies confirmed the presence of loaded organic compounds on the SBA-15 surface as shown in Figure 6. In two TGA curves, the weight loss (up to 150 • C) was due to physisorbed water from the mesoporous material. The obtained results pointed out that SBA-Pr-Is-TAP is hydrophilic because of the existence of more nitrogen atoms. The weight loss at 200 and 650 • C was related to the breakdown and removal of organic compounds while that at 700 • C corresponds to the dehydroxylation of silanol groups. TGA curves indicated, respectively, weight losses of ∼6% (0.76 mmol), ∼16% (0.845 mmol), and 24% (0.809 mmol) of organic compounds for SBA-Pr-Cl, SBA-Pr-Is, and SBA-Pr-Is-TAP as expected for materials with increasing loadings of organic content.  Thermogravimetry studies confirmed the presence of loaded organic compounds on the SBA-15 surface as shown in Figure 6. In two TGA curves, the weight loss (up to 150 °C) was due to physisorbed water from the mesoporous material. The obtained results pointed out that SBA-Pr-Is-TAP is hydrophilic because of the existence of more nitrogen atoms. The weight loss at 200 and 650 °C was related to the breakdown and removal of organic compounds while that at 700 °C corresponds to the dehydroxylation of silanol groups. TGA curves indicated, respectively, weight losses of ∼6% (0.76 mmol), ∼16% (0.845 mmol), and 24% (0.809 mmol) of organic compounds for SBA-Pr-Cl, SBA-Pr-Is, and SBA-Pr-Is-TAP as expected for materials with increasing loadings of organic content.
This high selectivity can be interpreted due to the different charge densities of metal ions and their interaction between guest and host. The charge density (ρ) of a metal ion is defined as the amount of electric charge/unit volume [36,37]. It is one of the most important parameters of the relative electrophilicity of a metal ion for interaction between
The results indicate that the charge density of Fe ion is the largest among all of competition ions. Therefore, the electrophilic ability (capability of forming a coordination complex with an electron-rich ligand) of an Fe 3+ ion is the strongest among cations. On the other hand, metal ions (as guest) can diffuse into the functionalized nanochannels of SBA-15 (as a host) [38]. The host-guest interaction depends on the concentration of ion and chelating capability into the channels, with Fe 3+ ions having an enhanced interaction with the strong electrophilicity of amine groups (TAP).

Figure 7.
Fluorescence emission of SBA-Is-TAP (3 mL H2O suspension, 0.2 g L − ¹) by different cations including Na + , Mg 2+ , Al 3+ , K + , Ca 2+ , Mn 2+ , Cd² + , Co 2+ , Cu 2+ , Li + , Zn 2+ , Ni² + , Ba² + , Hg² + , Sr² + , Cs + , Pb 2+ , Fe 2+ , Cr 3+ , Ag + , and Fe 3+ (100 µL, 1 × 10⁻ 4 M, λex = 220 nm and λem = 420 nm). The results indicate that the charge density of Fe 3+ ion is the largest among all of competition ions. Therefore, the electrophilic ability (capability of forming a coordination complex with an electron-rich ligand) of an Fe 3+ ion is the strongest among cations. On the other hand, metal ions (as guest) can diffuse into the functionalized nanochannels of SBA-15 (as a host) [38]. The host-guest interaction depends on the concentration of ion and chelating capability into the channels, with Fe 3+ ions having an enhanced interaction with the strong electrophilicity of amine groups (TAP). This high selectivity can be interpreted due to the different charge densities of metal ions and their interaction between guest and host. The charge density (ρ) of a metal ion is defined as the amount of electric charge/unit volume [36,37]. It is one of the most important parameters of the relative electrophilicity of a metal ion for interaction between ligand and ion. The charge densities of metal ions were calculated according to Equation (1): where q is the formal charge and r denotes the Shannon ionic radius [38]. The calculated charge density of the cations increase in the following order: Al 3+ (4.67) < Cr 3+ (3.08) < Fe 3+ (6.45).
The results indicate that the charge density of Fe 3+ ion is the largest among all of competition ions. Therefore, the electrophilic ability (capability of forming a coordination complex with an electron-rich ligand) of an Fe 3+ ion is the strongest among cations. On the other hand, metal ions (as guest) can diffuse into the functionalized nanochannels of SBA-15 (as a host) [38]. The host-guest interaction depends on the concentration of ion and chelating capability into the channels, with Fe 3+ ions having an enhanced interaction with the strong electrophilicity of amine groups (TAP).
Selectivity studies of SBA-Pr-Is-TAP towards Fe 3+ ions were subsequently conducted in the presence of other cations (Figure 8). Spectra were recorded at λ ex = 220 nm and λ em = 420 nm after the addition of SBA-Pr-Is-TAP suspension (0.02 g L −1 , 3 mL H 2 O) to the 100 µL of Fe 3+ mixture (1 × 10 −4 M) with 100 µL (1 × 10 −4 M) of the mentioned metal cations. Remarkably, results indicated that SBA-Pr-Is-TAP has good selectivity for Fe 3+ among a range of different tested cations.  A fluorescence titration experiment was also carried out with the addition of various amounts of Fe³⁺ ions (1 × 10 −4 to 100 × 10 −4 M) into the aqueous suspension of SBA-Pr-Is-TAP. The relation between fluorescence intensity and different concentrations of Fe³⁺ is shown in Figure 9. Fluorescence intensity was enhanced by increasing Fe³⁺ concentrations. A linear relation between fluorescence intensity and concentration of Fe³⁺ was observed, with R 2 of 0.9991 as shown in Figure 10. The detection limit was calculated according to the following equation: DL = KSd/m. DL was calculated to be 2.43 × 10 −6 , where Sd is the standard deviation of the blank solution measured five times, m is the slope of fluorescence intensity of SBA-Pr-Is-TAP versus [Fe³⁺], and K is 3 as a confidence level. It showed good detection of limit in comparison with the other fluorescent compounds in Table 2.  A fluorescence titration experiment was also carried out with the addition of various amounts of Fe 3+ ions (1 × 10 −4 to 100 × 10 −4 M) into the aqueous suspension of SBA-Pr-Is-TAP. The relation between fluorescence intensity and different concentrations of Fe 3+ is shown in Figure 9. Fluorescence intensity was enhanced by increasing Fe 3+ concentrations. A linear relation between fluorescence intensity and concentration of Fe 3+ was observed, with R 2 of 0.9991 as shown in Figure 10. The detection limit was calculated according to the following equation: DL = KS d /m. DL was calculated to be 2.43 × 10 −6 , where S d is the standard deviation of the blank solution measured five times, m is the slope of fluorescence intensity of SBA-Pr-Is-TAP versus [Fe 3+ ], and K is 3 as a confidence level. It showed good detection of limit in comparison with the other fluorescent compounds in Table 2.
Fluorescence studies of SBA-Pr-Is-TAP after adding different anions A n− , including = I − , 2− , OH − , and Cr 2 O 7 2− , was obtained at λ ex = 220 nm, λ em = 420 nm as shown in Figure 11 and Scheme 3. Luminescence intensities were influenced by adding Cr 2 O 7 2− ions as compared to other anions. Therefore, SBA-Pr-Is-TAP also showed a potential recognition and sensing for Cr 2 O 7 2− ions.  Fluorescence studies of SBA-Pr-Is-TAP after adding different anions Aⁿ⁻, includ I -, CN -, Cl -, F -, Br -, CO3 2-, SO4 2-, HCO3 -, H2PO4 -, NO2 -, NO3 -, CH3COO -, SCN -, S2O3 2-, OH Cr₂O₇²⁻, was obtained at λex = 220 nm, λem = 420 nm as shown in Figure 11 and Sche Luminescence intensities were influenced by adding Cr₂O₇²⁻ ions as compared to anions. Therefore, SBA-Pr-Is-TAP also showed a potential recognition and sensin Cr₂O₇²⁻ ions.  Fluorescence studies of SBA-Pr-Is-TAP after adding different anions Aⁿ⁻, including = I -, CN -, Cl -, F -, Br -, CO3 2-, SO4 2-, HCO3 -, H2PO4 -, NO2 -, NO3 -, CH3COO -, SCN -, S2O3 2-, OH -, and Cr₂O₇²⁻, was obtained at λex = 220 nm, λem = 420 nm as shown in Figure 11 and Scheme 3. Luminescence intensities were influenced by adding Cr₂O₇²⁻ ions as compared to other anions. Therefore, SBA-Pr-Is-TAP also showed a potential recognition and sensing for Cr₂O₇²⁻ ions.   The selectivity of SBA-Pr-Is-TAP towards Cr₂O₇²⁻ ions was subsequently investigated in the presence of other anions (Figure 12). In this regard, spectra were recorded at λₑₓ = 255 nm and λₑₘ = 420 nm after the addition of SBA-Pr-Is-TAP suspension (0.02 g L − ¹, 3 mL H2O) to the mixture of Cr₂O₇²⁻ (100 µL, 1 × 10 −4 M) and the mentioned anions (100 µL, 1 × 10 −4 M). The obtained results showed that the SBA-Pr-Is-TAP has good selectivity for Cr₂O₇²⁻. 2− is shown in Figure 13. Fluorescence intensity decreased while the concentration of Cr 2 O 7 2− ions gradually increased. A linear relation between fluorescence intensity and concentration of Cr 2 O 7 2− was found, with R 2 of 0.9941 as shown in Figure 14. The detection limit was calculated (similarly to that for Fe 3+ ) to be 3.96 × 10 −7 M and compared with the other chemosensor compounds in Table 3.  Figure 13. Fluorescence intensity decreased while the concentration of Cr₂O₇²⁻ ions gradually increased. A linear relation between fluorescence intensity and concentration of Cr₂O₇²⁻ was found, with R 2 of 0.9941 as shown in Figure 14. The detection limit was calculated (similarly to that for Fe 3+ ) to be 3.96 × 10 −7 M and compared with the other chemosensor compounds in Table 3.   The quenching mechanism of SBA-Pr-Is-TAP with Cr₂O₇²⁻ was subsequently investigated using the Stern-Volmer equation. Figure 14 shows a linear plot of I0/I against the concentration of Cr₂O₇²⁻ ions which indicates a static quench. UV-Vis spectrum of Cr₂O₇²⁻ ions exhibits two maximum absorption wavelengths at 257 and 350 nm in UV range and another at 440 nm in the visible domain which has good spectral overlap with the fluorescence emission peak of SBA-Pr-Is-TAP [47]. Therefore, the strong inner filter effect (IFE) possibly occurs after the addition of Cr₂O₇²⁻ in the dispersed SBA-Pr-Is-TAP [48,49]. In several reports, IFE has been proposed as a fluorescence quenching mechanism in optical

Conclusions
SBA-Pr-Is-TAP as organic-inorganic nanohybrid via SBA-15 modification with isatin and 2,4,6-triaminopyrimidine was synthesized and characterized. Textural and physicochemical properties confirmed the successful incorporation of organic groups on SBA-15. Fluorescence studies demonstrated the hybrid or an organic material can be a selective sensor for Fe³⁺ and Cr₂O₇²⁻ recognition in H2O, with detection limits of 2.43 × 10 −6 and 3.96 × 10 −7 M, respectively.

Synthesis of SBA-Pr-Is-TAP
SBA-15-Pr-Is (1 g) was dispersed in dry EtOH. 2,4,6-triaminopyrimidine (0.62 g, 5 mmol) was added to the mixture of reactions. It was refluxed for 24 h, and then filtered to obtain the dark brown powder, which was soxhlated with dried EtOH to obtain pure SBA-15-Pr-Is.
N2 adsorption-desorption isotherms were obtained using a BELSORP-miniII instrument at liquid nitrogen temperature (−196 °C). Degassing process for all samples was performed at 100 °C before the measurements. The Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) equations were applied on sorption data using BELSORP analysis software to calculate the physical properties of materials such as the specific surface area, pore diameter, pore-volume, and pore size distribution.
Thermogravimetric analyses (TGA) were carried out by a TGA Q50 V6.3 Build 189 instrument from ambient temperature to 1000 °C with a ramp rate of 10 ℃ min −1 in the air. SEM analysis was performed on a Hitachi S-4160 operated at 30 kV.
FT-IR spectra were obtained in KBr disks on a RAYLEIGH WQF-510A FT-IR spectrometer in the 600-4000 cm −1 region. Fluorescence measurements were collected on a Cary Eclipse Fluorescent Spectrophotometer.  The quenching mechanism of SBA-Pr-Is-TAP with Cr 2 O 7 2− was subsequently investigated using the Stern-Volmer equation. Figure 14 shows a linear plot of I 0 /I against the concentration of Cr 2 O 7 2− ions which indicates a static quench. UV-Vis spectrum of Cr 2 O 7 2− ions exhibits two maximum absorption wavelengths at 257 and 350 nm in UV range and another at 440 nm in the visible domain which has good spectral overlap with the fluorescence emission peak of SBA-Pr-Is-TAP [47]. Therefore, the strong inner filter effect (IFE) possibly occurs after the addition of Cr 2 O 7 2− in the dispersed SBA-Pr-Is-TAP [48,49]. In several reports, IFE has been proposed as a fluorescence quenching mechanism in optical dichromate sensors [18,50]. This quenching mechanism was proposed based on the formation of hydrogen bonding between the oxygen group of Cr 2 O 7 2− and hydrogen atoms belong to the amine or carboxyl group of functionalized TAP into the nanochannel of SBA-15 [51]. This hydrogen bonding may change the electronic distribution of TAP; thereby, the fluorescence emissions can be quenched and intensity changed. Therefore, besides IFE, hydrogen bonding is suggested as fluorescence quenching mechanism of SBA-Pr-Is-TAP with Cr 2 O 7 2− .

Conclusions
SBA-Pr-Is-TAP as organic-inorganic nanohybrid via SBA-15 modification with isatin and 2,4,6-triaminopyrimidine was synthesized and characterized. Textural and physicochemical properties confirmed the successful incorporation of organic groups on SBA-15. Fluorescence studies demonstrated the hybrid or an organic material can be a selective sensor for Fe 3+ and Cr 2 O 7 2− recognition in H 2 O, with detection limits of 2.43 × 10 −6 and 3.96 × 10 −7 M, respectively.

Synthesis of SBA-Pr-Is-TAP
SBA-15-Pr-Is (1 g) was dispersed in dry EtOH. 2,4,6-triaminopyrimidine (0.62 g, 5 mmol) was added to the mixture of reactions. It was refluxed for 24 h, and then filtered to obtain the dark brown powder, which was soxhlated with dried EtOH to obtain pure SBA-15-Pr-Is.
Low-angle X-ray scattering measurements were performed using an X'Pert Pro MPD diffractometer using Cu K α radiation (λ = 1.5418Ǻ). N2 adsorption-desorption isotherms were obtained using a BELSORP-miniII instrument at liquid nitrogen temperature (−196 • C). Degassing process for all samples was performed at 100 • C before the measurements. The Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) equations were applied on sorption data using BELSORP analysis software to calculate the physical properties of materials such as the specific surface area, pore diameter, pore-volume, and pore size distribution.
Thermogravimetric analyses (TGA) were carried out by a TGA Q50 V6.3 Build 189 instrument from ambient temperature to 1000 • C with a ramp rate of 10°C min −1 in the air. SEM analysis was performed on a Hitachi S-4160 operated at 30 kV.
FT-IR spectra were obtained in KBr disks on a RAYLEIGH WQF-510A FT-IR spectrometer in the 600-4000 cm −1 region. Fluorescence measurements were collected on a Cary Eclipse Fluorescent Spectrophotometer.
Additional information can be found in the Supplementary Materials submitted with this article.

Calculation of Detection Limit (DL)
DLs can be calculated based on the standard deviation of the response (S d ) of the curve and the slope of the calibration curve (m) at levels approximating the DL according to the formula: DL = 3 (S d /m). The standard deviation of the response can be determined based on the standard deviation of y-intercepts of regression lines. The slope and S d can be obtained with one order of magnitude of the calibration curve.