SBA-15 Functionalized with Naphthalene Derivative for

ANALYTICAL SCIENCES MAY 2016, VOL. 32 511

Introduction

Chromium(VI) is a stable form of chromium chemicals that can cause deleterious human health effects,1 such as lung cancer, respiratory irritation, dermatosis, and kidney and liver damage. It spreads into the environment through metal plating, paint pigments, and leather tanning.2 In addition, chromium enters natural water by the weathering of chromium-containing rocks and leaching from soils. Up to now, a wide range of traditional methods have been established for monitoring chromium compounds, such as atomic absorption spectrometry3 and high-performance liquid chromatography with diode-array detection.4 But their utilizations are limited due to the complicated procedure and expensive apparatus.

Nowadays, a new emerging type of analytical method, i.e. using fluorescent chemical sensors, has been successfully proven to be an excellent alternative for the aforementioned techniques in terms of simplicity, selectivity, sensitivity, and fast response time for analyzing trace levels of hazardous pollutants.5–7 Fluorescent chemical sensors consist of a fluorophore as a signaling moiety linked to an ionophore as a binding site wherein the signaling moiety converts a recognition event into an optical signal expressed as changes in the fluorescence emission characteristics of fluorophore. In this field, a numerous successful examples of such sensors have been reported for the detection of various cations and anions.8–12 As a new category of these tools, namely hybrid optical sensors, fluorophore and ionophore have been recently incorporated onto the surface of nanoscopic inorganic matrices, such as TiO2,

SiO2, gold nanoparticle, and ordered mesoporous silica materials (OMS).13–16 These hybrid optical sensors possess a few more advantages over other types of fluorescent chemical sensors, such as long-life durability, reversibility and direct applicability in water, which make them more suitable for real-world sensing applications. Among these inorganic matrices, the SBA-15 type of OMS material has become the most commonly used one due to its high specific surface area which allows a great density of fluorophores to be hosted onto its surface. A large pore diameter and straight cylindrical pores facilitates the diffusion of targets through its channels, high hydrothermal stability, and biocompatibility.17–22 In addition, because of the availability of abundant –OH groups on the surface of SBA-15 and a variety of alkoxysilane linkers, various fluorophores, such as 8-hydroxyquinoline,23 dansylamide,24 pyrene,25 and 1,8-naphthalimide26 can be easily attached onto its surface. Furthermore, optical transparency, photophysically inertness, and photostability of SBA-15, make it an excellent candidate to be exploited in preparing hybrid optical sensors with the advantages of simple-handling and storage. Despite these very promising features, limited examples have been reported about using OMS material for sensing anions.23,27–30 Also, to the best of our knowledge, there is only one report about sensing dichromate anion based on this material.31 In this work, Hosseini et al. reported an optical sensor created by the assembly of a fluorescent aluminum complex of 8-hydroxy-quinoline (AlQx) within the channels of modified SBA-15 for the recognition of dichromate anions in water, based on the quenching fluorescence emission of a complex resulting from photo-induced electron transfer.

In this paper, we utilized these advantages to prepare a hybrid optical sensor by the covalent immobilization of a naphthalene derivative onto the surface of SBA-15 by a simple post-grafting

2016 © The Japan Society for Analytical Chemistry

† To whom correspondence should be addressed.E-mail: [email protected].

SBA-15 Functionalized with Naphthalene Derivative for Selective Optical Sensing of Cr2O7

2– in Water

Mehdi KARIMI,* Alireza BADIEI,*,**† and Ghodsi MOHAMMADI ZIARANI***

*School of Chemistry, College of Science, University of Tehran, Tehran, Iran ** Nanobiomedicine Center of Excellence, Nanoscience and Nanotechnology Research Center,

University of Tehran, Tehran 98-21, Iran *** Department of Chemistry, Faculty of Science, Alzahra University, Tehran 98-21, Iran

A novel organic-inorganic hybrid optical sensor (NUS) was designed and prepared in two steps: the grafting of 3-(isocyanatopropyl)trimethoxysilane onto the surface of SBA-15, and then the attachment of naphthalene-1-amine. The obtained materials were characterized using low-angle XRD, N2 adsorption-desorption, TGA, FT-IR, and TEM techniques. A fluorescence study revealed that the NUS was a highly selective optical sensor for the detection of Cr2O7

2– among various anions, including F–, I–, Cl–, Br–, CO3

2–, HCO3–, NO3

–, H2PO4–, CH3COO–, and NO2

– with a good linearity between the fluorescence intensity and the concentration of Cr2O7

2– as well as a detection limit of 1.2 × 10–7 M in a 100% aqueous medium. Moreover, the applicability of real samples of the sensor is also discussed.

Keywords Ordered mesoporous silica, SBA-15, fluorescence, optical sesnor, Cr2O72–

(Received August 27, 2015; Accepted November 27, 2015; Published May 10, 2016)

512 ANALYTICAL SCIENCES MAY 2016, VOL. 32

method. In this regard, 3-(isocyanatopropyl)-trimethoxysilane was firstly immobilized on the surface of SBA-15, and then naphthalene-1-amine was attached to this functional group. This sensor had been previously prepared by a different technique in one step by our research group, and was tested over various cations. It showed selectivity for sensing Fe3+ in water.32 The prepared sensor in this work was prepared in two step and evaluated toward various anions. It exhibited a high selectivity and sensitivity for the detection of dichromate ions in water.

Materials and Methods

MaterialsTetraethyl orthosilicate (TEOS, Merck), 1-naphthylamine

(Sigma Aldrich), THF (Merck) and nitrate salts of the metal cations (Sigma Aldrich), Pluronic P123 (EO20PO70EO20, MW = ca. 5800, Sigma Aldrich) and 3-(isocyanatopropyl)trimethoxy-silane. All of the above materials were used without any further purification. Deionized water was used in all procedures.

MethodsLow-angle X-ray scattering measurements were performed on

an X’Pert Pro MPD diffractometer using Cu Kα radiation (λ = 1.5418 Å). N2 adsorption-desorption isotherms were obtained using a BELSORP-miniII instrument at liquid-nitrogen temperature (–196°C). All samples were degassed at 100°C before performing 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. The Fourier transform infrared (FT-IR) spectra of samples were recorded on a RAYLEIGH WQF-510A apparatus. Transmission electron microscopy (TEM) was performed on a Zeiss EM900 instrument at an accelerating voltage of 80 kV. Samples were dispersed in ethanol using an ultrasonic bath. A drop of the ethanol mixture was placed on a carbon-coated copper grid for analysis. Thermogravimetric analysis (TGA) was carried out in a TGA Q50 V6.3 Build 189 instrument from ambient temperature to 1000°C with a ramp rate of 15°C/min in air. Fluorescence spectra were recorded on an Agilent G980A instrument.

Preparation of SBA-15SBA-15 was prepared according to a Ref. 33. In a cylindrical

container, a mixture of 11.7 g P123, 303.4 g water and 73.3 g HCl 37% was slowly stirred until a homogenous solution was prepared. Then, the temperature was adjusted to 55°C and mixture was stirred for 3 h. Next, 25 g of TEOS was added under rigorous stirring, and the mixture remained at the static condition for 24 h at 55°C. The reaction batch was subsequently transferred to an autoclave and kept in an oven for another 24 h at 100°C. The product was washed with a HCl/ethanol mixture, and finally calcined for 3 h at 550°C.

Preparation of alkyl-naphthalene-urea immobilized on SBA-15 (NUS)

First 2 g of dried SBA-15 was dispersed in 100 ml of toluene for 30 min. Then, 10 mmol 3-(isocyanatopropyl)trimethoxy-silane was added, and the mixture was refluxed for 24 h. After that, the resulting solid was filtered, washed several times with an excess amount of toluene for removing the unreacted 3-(isocyanatopropyl)trimethoxysilane, and then dried overnight. Subsequently, a mixture of 1 g of the isocyanatopropyl-

functionalized SBA-15 plus 5 mmol naphthalene-1-amine in THF was refluxed for 6 h. The final pale-pink solid (alkyl-naphthalene-urea immobilized SBA-15 abbreviated as NUS) was filtered, and washed with an excess amount of THF and ethanol. Figure 1 provides the overall synthesis procedure of NUS.

Procedure for a fluorescence measurementThe sensor (0.2 g L–1) was dispersed into deionized water by

an ultrasonic bath. Stock solutions of anions (1 × 10–2 M) were prepared by dissolving their sodium salts in deionized water. Florescence spectra were recorded 2 min after mixing of the sensor (3 ml) and the anions (25 μL, 1 × 10–2 M). In competition fluorescence experiments, 3 ml of the sensor was added into a mixture of 10 μl of Cr2O7

2– + 50 μl other anions in a cuvette. For fluorescence titration experiments, 3 ml of the sensor filled in the cuvette and the concentration of the Cr2O7

2– was increased by the stepwise addition of different concentrations. Excitation wavelength was 290 nm, and both the excitation and emission slit widths were set to 5 nm. The pH of in all of the experiments was about 7. In order to avoid errors resulting from volume changes, the total volume of added anions was kept below 100 μl.

Results and Discussion

Low-angle XRDLow-angle XRD patterns of original SBA-15 and

functionalized SBA-15 are depicted in Fig. 2. Typically, a mesoporous silica material exhibits three distinctive diffractions: one located near the 2θ value of 1° related to the diffraction from the plane of (100) and two others located near 2θ of 1.8°, related to the diffractions from planes of (110) and (200). The presence of these three diffractions together implied the high order in the hexagonal mesoporous structure of the sample. As can be seen, both samples showed these three diffractions, which indicated that the mesoporous structure of SBA-15 was preserved after modification steps. However, a decrease in the fluorescence intensity of the NUS resulted from the attachment of organic moieties onto the walls of SAB-15. Moreover, a TEM image of SBA-15 is shown in the inset of Fig. 2. It further clearly exhibits the ordered channels that run along the length direction of the rods.

Fig. 1　Overall synthetic procedure of NUS.


N2 adsorption-desorptionN2 adsorption-desorption isotherms of SBA-15 and NUS are

provided in Fig. 3. Both samples exhibited isotherms (type IV) with typical H1 hysteresis, verifying the mesoporous structures of the samples again. The presence of this feature in functionalized NUS suggested that the pores were still open, and were not blocked during functionalization steps. However, the lowered volume adsorbed in NUS sample was due to the incorporation of organic moeites. As listed in the inset of Fig. 3, the reduced quantity of the structural parameters, including the specific surface area (SBET), average pore diameter (da), and average pore volume (Va) in NUS further confirms the incorporation of the silanol and naphthalene moieties.

FT-IRThe FT-IR spectra shown in Fig. 4 proved the successful

attachment of organic moieties onto the surface of SBA-15. In all samples, the peaks located at around 800, 960, 1100 cm–1 resulted from symmetric stretching vibrations of Si–O,

symmetric stretching vibration of Si–OH, and asymmetric stretching vibrations of Si–O–Si, respectively. The peak around 1640 cm–1 was attributed to the physically absorbed water molecules. The wide peak with a maximum located at 3500 cm–1 in Fig. 4a was due to asymmetric stretching vibrations of the –OH groups. The intensity of this peak for the functionalized samples in Figs. 4b and 4c was reduced due to the attachment of organic moieties to the Si–OH groups. Moreover, the peaks at around 1530 and 2790 – 2960 cm–1 in both Figs. 4b, 4c were related to the C=O vibrations and stretching vibrations of the –CH2– groups of propyl chains, respectively. In Fig. 4c, the peaks located at 1370 and 1460 cm–1 were, respectively, due to the C=N and C=C aromatic stretching vibrations, which confirmed attachment of the naphthalene moiety.

TGAFigure 5 gives a thermogravimetric analysis of isocyanatopropyl-

functionalized SBA-15 and NUS. The weight loss up to 150°C was attributed to the elimination of physisorbed water molecules; above that, it was attributed to the decomposition of organic moieties. As is shown in Fig. 5, the estimated weight-loss for isocyanatopropyl-functionalized SBA-15 and NUS was about 8 and 14%, respectively. The weight-loss differences between

Fig. 2　Low-angle XRD patterns of SBA-15 and NUS. The inset is a TEM image of as-synthesized SBA-15.

Fig. 3　N2 adsorption-desorption isotherms of (a) SBA-15, (b) NUS. The inset is the table of structural parameters.

Fig. 4　FT-IR spectra of (a) SBA-15 and (b) isocyanate-functionalized SBA-15 and (c) NUS.

Fig. 5　Thermogravimetric analysis of (a) isocyanate-functionalized SBA-15 and (b) NUS.


these two samples could be assigned to the attached 1-naphthylamine groups. Therefore, the estimated amount of attached 1-naphthylamine to the surface in NUS sample was about 0.036 mol g–1.

Fluorescence properties of NUSThe fluorescence spectrum of free dispersed NUS exhibited a

broad emission with a maximum value located at 380 nm, following an excitation at 290 nm. To evaluate the recognition ability of dispersed NUS towards anions, the changes in the fluorescence intensity were individually recorded upon the addition of I–, Cl–, Br–, CO3

2–, HCO3–, NO3

–, H2PO4–, NO2

–, Cr2O7

2–, and CH3COO– to the NUS. As can be clearly seen in Fig. 6, negligible or no change in the fluorescence intensity was observed for all anions, except for Cr2O7

2–, which induced effective fluorescence quenching about of 30% compared to that of other anions. Figure 6 demonstrates the proposed binding mode of the NUS[Cr2O7

2–] system. Cr2O72– binds to the urea

functional groups via hydrogen binding to form a stable complex. The quenching mechanism could be due to a photon-

induced electron transfer (PET) process from Cr2O72– to the

photoexcited state of the naphthalene unit.34

High selectivity towards Cr2O72– ions over other competitive

anions is an important feature of optical sensors. The illustrated results in Fig. 7 from competitive experiments implied that insignificant changes occurred in the presence of other anions, and that Cr2O7

2– gave rise to the same quenching effect in all cases, thus confirming that NUS could be considered as a highly selective optical sensor for Cr2O7

2–, even in presence of higher amounts of other anions (5-fold) as interfering agents.

For practical applications, it is important that the optical sensor be capable of detecting Cr2O7

2– over a wide range of operational pH values. The changes in the fluorescence intensities of NUS were recorded in the absence and presence of Cr2O7

2– as a function of the pH, ranging from pH = 2 to 12. As can be seen in Fig. 8, the sensing ability of NUS was independent from the pH, suggesting that no buffer solution is required for the detection of Cr2O7

2– ions.Fluorescence titraton was carried out in order to investigate

the detection ability of the NUS for Cr2O72– ions following

excitation at 290 nm. As can be seen from Fig. 9, in the absence of Cr2O7

2–, the intense fluorescence emitted by the NUS was gradually quenched upon the addition of an increasing concentration of Cr2O7

2–. Moreover, the plot of I0/I against the

Fig. 6　Fluorescence emission spectra NUS (0.2 g L–1) (λex = 290 nm). The inset is depicted binding mode of Cr2O7

2– with NUS (λex = 290 nm).

Fig. 7　(a) Gray bars represent the normalized fluorescence intensity of the aqueous suspended NUS (0.2 g L–1) upon the addition of anions. (b) White bars represent the normalized fluorescence intensity of NUS (0.2 g L–1) in the presence of Cr2O7

2– (10 μL) and other anions as interfering agents (50 μL) (λex = 290 nm).

Fig. 8　Effect of the pH on the fluorescence intensity of NUS (0.2 g L–1) in the absence and presence of Cr2O7

2– (λex = 290 nm).

Fig. 9　Fluorescence titration of NUS (0.2 g L–1) in the presence of increasing concentration of Cr2O7

2– (λex = 290 nm).


quencher concentration in Fig. 10, constructed based on the fluorescence titration results, reavls a good linearity between I0/I and the Cr2O7

2– concentrations at lower concentrations. Here, I0 and I are the fluorescence intensity of the NUS before and after the addition of the Cr2O7

2– solution, respectively. It follows the equation of I0/I = 0.87 × 105[Cr2O7

2–]+ 0.99 with a linearly dependent coefficent, R2, of 0.9984.

Regarding the Stern–Volmer equation, I0/I = 1 + KSV[Q], the Stern–Volmer quenching constant (KSV) was 6 × 105 M–1. KSV reflects the accessibility of the fluorophores to the quencher. The detection limit (DL) was calculated based on DL = 3Sd/m, where Sd represents the standard deviation of the blank solution measured by 10 times; m represents the slope of the fluorescence intensity versus the concentration of Cr2O7

2–. Therefore, DL was calculated to be 1.2 × 10–2 M, which is compred with those reported in Refs. 3 and 4 in Table 1.

The applicability of the NUS for the detection of Cr2O72– in

real samples was evaluated. Also, the experiments were carried out based on the proposed method in Ref. 31. The sensor was tested in three different samples, including drinking water, tap water and waste water (Electroplating Industrial of Tehran). As the results shown in Table 2, the proposed method successfully

determined Cr2O72– with good precision using a calibration

curve and a good relative standard deviation (RSD), calculated by of three replicate determinations.

Conclusion

In summary, a novel organic-inorganic hybrid material based on the functionalization of SBA-15 with a naphthalene derivative (NUS) was synthesized and characterized using XRD, N2 adsorption-desorption, TGA, FT-IR, and TEM techniques. Fluorescence experiments revealed that NUS was able to detect Cr2O7

2– anion selectively over other competitive anions in water. Moreover, a pH effect evaluation showed that varying the pH value from 2 to 12 did not disturb the anion-recognition ability. The detection limit was calculated to be 1.2 × 10–7 M. Moreover, the real sample applicability of the sensor was confirmed in three different real samples.

Acknowledgements

This work was supported by the research council of University of Tehran.

References

1. O. García-Beltrán, N. Mena, O. Yañez, J. Caballero, V. Vargas, M. T. Nuñez, and B. K. Cassels, Eur. J. Med. Chem., 2013, 67, 60.

2. D. E. Kimbrough, Y. Cohen, A. M. Winer, L. Creelman, and C. Mabuni, Crit. Rev. Env. Sci. Tech., 1999, 29, 1.

3. T. Prasada Rao, S. Karthikeyan, B. Vijayalekshmy, and C. S. P. Iyer, Anal. Chim. Acta, 1998, 369, 69.

4. S. Cathum, C. Brown, and W. Wong, Anal. Bioanal. Chem., 2002, 373, 103.

5. A. P. De Silva, H. N. Gunaratne, T. Gunnlaugsson, A. J. Huxley, C. P. McCoy, J. T. Rademacher, and T. E. Rice, Chem. Rev., 1997, 97, 1515.

6. B. Valeur and I. Leray, Coord. Chem. Rev., 2000, 205, 3. 7. R. Martínez-Máñez and F. Sancenón, Chem. Rev., 2003,

103, 4419. 8. M. Wenzel, J. R. Hiscock, and P. A. Gale, Chem. Soc. Rev.,

2011, 41, 480. 9. H. N. Kim, M. H. Lee, H. J. Kim, J. S. Kim, and J. Yoon,

Chem. Soc. Rev., 2008, 37, 1465. 10. H. N. Kim, W. X. Ren, J. S. Kim, and J. Yoon, Chem. Soc.

Rev., 2012, 41, 3210. 11. M. Karimi, A. Badiei, and G. Mohammadi Ziarani, RSC

Adv., 2015, 5, 36530. 12. M. Karimi, A. Badiei, N. Lashgari, J. Afshani, and G.

Mohammadi Ziarani, J. Lumin., 2015, 168, 1. 13. E. Palomares, R. Vilar, A. Green, and J. R. Durrant, Adv.

Funct. Mater., 2004, 14, 111. 14. S. O. Obare, R. E. Hollowell, and C. J. Murphy, Langmuir,

2002, 18, 10407. 15. X. Tian, Z. Dong, R. Wang, and J. Ma, Sens. Actuators, B,

2013, 183, 446. 16. X. Guo, B. Li, L. Zhang, and Y. Wang, J. Lumin., 2012,

132, 1729. 17. M. Yadavi, A. Badiei, and G. M. Ziarani, Appl. Surf. Sci.,

2013, 279, 121. 18. P. Zarabadi-Poor, A. Badiei, A. A. Yousefi, and J. Barroso-

Flores, J. Phys. Chem. C, 2013, 117, 9281.

Fig. 10　Linear relationship between the normalized fluorescence intensity of NUS (0.2 g L–1) and concentration of Cr2O7

2– (λex = 290 nm).

Table 1　Comparison of the detection limits (DL) of different methods for the determination of Cr2O7

2–

Method Detection limit (DL) Reference

Fluorescent optical sensor 0.025 mg L–1 This workFlame atomic-absorption spectrometry

0.02 μg L–1 3

High-performance liquid chromatography

4 mg L–1 4

Table 2　Determination of Cr2O72– in different environmental

water samples

Water sampleAdded

amount/μMFound

amount/μMRecovery (100%)

RSD, %a

Drinking water 0.25 0.251 ± 0.01 100.4 2.48Tape water 0.25 0.254 ± 0.013 101.6 3.56Waste water (industrial) 0.25 0.258 ± 0.015 103.2 4.32

a. Relative standard deviation.


19. H. Kim, S. Lee, S. Lee, M. Seo, and J. Jung, Chem. Commun., 2008, 3921.

20. Z. Dong, X. Tian, Y. Chen, J. Hou, and J. Ma, RSC Adv., 2013, 3, 2227.

21. T. Balaji, M. Sasidharan, and H. Matsunaga, Analyst, 2005, 130, 1162.

22. C. Song, X. Zhang, C. Jia, P. Zhou, X. Quan, and C. Duan, Talanta, 2010, 81, 643.

23. M. R. Ganjali, V. K. Gupta, M. Hosseini, Z. Rafiei-Sarmazdeh, F. Faridbod, H. Goldooz, A. R. Badiei, and P. Norouzi, Talanta, 2012, 88, 684.

24. X. Wang, and H. Yang, Appl. Surf. Sci., 2013, 277, 257. 25. Y. Wang, B. Li, L. Zhang, L. Liu, Q. Zuo, and P. Li, New J.

Chem., 2010, 34, 1946. 26. Q. Meng, X. Zhang, C. He, P. Zhou, W. Su, and C. Duan,

Talanta, 2011, 84, 53. 27. A. B. Descalzo, D. Jimenez, J. E. Haskouri, D. Beltran, P.

Amoros, M. D. Marcos, R. Martinez-Manez, and J. Soto,

Chem. Commun., 2002, 562. 28. A. B. Descalzo, D. Jimenez, M. D. Marcos, R. Martínez-

Máñez, J. Soto, J. El Haskouri, C. Guillém, D. Beltrán, P. Amorós, and M. V. Borrachero, Adv. Mater., 2002, 14, 966.

29. A. B. Descalzo, M. D. Marcos, R. Martinez-Manez, J. Soto, D. Beltran, and P. Amoros, Mater. Chem., 2005, 15, 2721.

30. R. Martínez-Máñez and F. Sancenón, Coord. Chem. Rev., 2006, 250, 3081.

31. M. Hosseini, V. K. Gupta, M. R. Ganjali, Z. Rafiei-Sarmazdeh, F. Faridbod, H. Goldooz, A. R. Badiei, and P. Norouzi, Anal. Chim. Acta, 2012, 715, 80.

32. M. Karimi, A. Badiei, and G. M. Ziarani, J. Fluoresc., 2012, 25, 1297.

33. Z. Bahrami, A. Badiei, and F. Atyabi, Chem. Eng. Res. Des., 2014, 92, 1296.

34. R. Martínez-Máñez and F. Sancenón, Chem. Rev., 2003, 103, 4419.

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SBA-15 Functionalized with Naphthalene Derivative for