Chemistry, Physics and Technology of Surface

ISSN 2518-1238 (Online), ISSN 2079-1704 (Print)

The purpose of this work is to study the possibility of creating hydrolytically and chemically stable pH-sensitive materials using silica as a carrier and an acid-base indicator that changes color when the pH of medium is varied. Synthesis of pH-sensitive materials was carried out by covalent binding of methyl red dye with aminopropylsilica in the presence of 1,1’-carbonyldiimidazole as a coupling agent. The porous structure, the chemical composition of the surface, and the protolytic properties of the synthesized silica materials were characterized by low-temperature adsorption-desorption of nitrogen, quantitative chemical analysis, thermogravimetry, diffuse reflectance electron spectroscopy, pH-metry. It was proved that chemical immobilization of methyl red on the silica surface does not have a significant effect on the porous structure of initial silica: the isotherms of nitrogen adsorption‑desorption and pore size distribution curves confirm retention of wide pore size distribution after chemical modification of silica. The content of the acid-base indicator chemically immobilized on the silica surface was determined by quantitative chemical analysis of synthesized materials, as well as by thermogravimetry and diffuse reflectance electron spectroscopy in the ultraviolet and visible regions. It was demonstrated that diffuse reflectance electron spectroscopy can be a reliable method of quantitatively determining an indicator dye chemically grafted to the silica surface. It was proved that chemically grafted dye retains the ability to protonation of tertiary amino group and formation of resonance azonium ion, which is responsible for changing the color of methyl red. It was found that the protonation of methyl red chemically fixed on the silica surface occurs at higher pH values than for individual indicator dye. Therefore, the color change of grafted dye takes place in less acidic solutions. The proposed synthetic approach can be used to produce regenerable and eco-friendly pH-sensitive materials.

1. Shamansky L.M., Yang M., Olteanu M., Chronister E.L. A spectroscopic study of the pH sensor response of fluorescein doped silica and aluminosilica sol-gel glasses. Mater. Lett. 1996. 26(3): 113. https://doi.org/10.1016/0167-577X(95)00214-6

2. Yang L., Saavedra S.S. Chemical sensing using sol-gel derived planar waveguides and indicator phases. Anal. Chem. 1995. 67(8): 1307. https://doi.org/10.1021/ac00104a002

3. Lee J.E., Saavendra S.S. Evanescent sensing in doped sol-gel glass films. Anal. Chim. Acta. 1994 285(3): 265. https://doi.org/10.1016/0003-2670(94)80064-2

4. Kraus S.C., Czolk R., Reichert J., Ache H.J. Optimization of the sol-gel process for the development of optochemical sensors. Sens. Actuators, B. 1993. 15–16(1–3): 199. https://doi.org/10.1016/0925-4005(93)85051-B

5. Bambot S.B., Sipior J., Lakowicz J.R., Rao G. Lifetime-based optical sensing of pH using resonance energy transfer in sol-gel films. Sens. Actuators, B. 1994. 22(3): 181. https://doi.org/10.1016/0925-4005(94)87019-5

6. Butler T.M., MacCraith B.D., McDonagh C. Leaching in sol-gel-derived silica film for optical pH sensing. J. Non-Cryst. Solids. 1998. 224(3): 249. https://doi.org/10.1016/S0022-3093(97)00481-X

7. Lee S.T., Gin J., Nampoori V.P.N., Vallabhan C.P.G., Unnikrishnan N.V., Radhakrishnan P. A sensitive fibre optic pH sensor using multiple sol-gel coatings. Journal of Optics A: Pure and Applied Optics. 2001. 3(5): 355. https://doi.org/10.1088/1464-4258/3/5/307

8. Wang E., Chow K.-F., Kwan V., Chin T., Wong C., Bocarsly A. Fast and long term optical sensors for pH based on sol-gels. Anal. Chim. Acta. 2003. 495(1–2): 45. https://doi.org/10.1016/S0003-2670(03)00904-8

9. Kowada Y., Ozeki T. Preparation of silica-gel film with pH indicators by the sol-gel method. J. Sol-Gel Sci. Technol. 2005. 33(2): 175. https://doi.org/10.1007/s10971-005-5612-7

10. Wu Z., Jiang Y., Xiang H., You L. Understanding the mechanisms of reaction and release of acid-base indicators entrapped in hybrid gels. J. Non-Cryst. Solids. 2006. 352(52–54): 5498. https://doi.org/10.1016/j.jnoncrysol.2006.09.008

11. Capeletti L.B., dos Santos J.H.Z., Moncada E. Quantification of indicator content in silica-based pH solid sensors by diffuse reflectance spectroscopy. Anal. Methods. 2011. 3: 2416. https://doi.org/10.1039/c1ay05235g

12. Capeletti L.B., Radtke C., dos Santos J.H.Z., Moncada E., Da Rocha Z.N., Pepe I.M. On the interaction of encapsulated pH indicator species within a silica matrix produced by three sol-gel routes. Colloids Surf., A. 2011. 392(1): 256. https://doi.org/10.1016/j.colsurfa.2011.10.002

13. Chauhan S.S., Jasra R.V., Sharma A.L. Dye immobilized mesoporous silica thin film as optical pH sensor. Int. J. Nanosci. 2012. 11(3): 1240001. https://doi.org/10.1142/S0219581X12400017

14. Zajic J., Traplova L., Matejec V., Pospisilova M., Barton I. Optical pH detection with U-shaped fiber-optic probes and adsorption transducers. Conference Papers in Science. 2015. 2015: 513621.

15. Yamaguchi A., Namekawa M., Kamijo T., Itoh T., Teramae N. Acid-base equilibria inside amine-functionalized mesoporous silica. Anal. Chem. 2011. 83(8): 2939. https://doi.org/10.1021/ac102935q

16. Oberg K.I., Hodyss R., Beauchamp J.L. Simple optical sensor for amine vapors based on dyed silica microspheres. Sens. Actuators, B. 2006. 115(1): 79. https://doi.org/10.1016/j.snb.2005.08.021

17. Nivens D.A., Zhang Y., Angel S.M. A fiber-optic pH sensor prepared using a base-catalyzed organo-silica sol-gel. Anal. Chim. Acta. 1998. 376(2): 235. https://doi.org/10.1016/S0003-2670(98)00505-4

18. Aigner D., Borisov S.M., Fernandez F.J.O., Sanchez J.F.F., Saf R., Klimant I. New fluorescent pH sensors based on covalently linkable PET rhodamines. Talanta. 2012. 99: 194. https://doi.org/10.1016/j.talanta.2012.05.039

19. Tertykh V.A., Belyakova L.A. Chemical reactions with the participation of silica surface. (Kiev: Naukova Dumka, 1991). [in Russian].

20. Belyakova L.A., Besarab L.N., Roik N.V., Lyashenko D.Yu., Vlasova N.N., Golovkova L.P., Chuiko A.A. Designing of the centers for adsorption of bile acids on a silica surface. J. Colloid Interface Sci. 2006. 294(1): 11. https://doi.org/10.1016/j.jcis.2005.06.081

21. Tawarah K.M., Abu-Shamleh H.M. A spectrophotometric study of the acid-base equilibria of o-methyl red in aqueous solutions. Dyes Pigm. 1991. 17(3): 203. https://doi.org/10.1016/0143-7208(91)80027-7

22. Kolthoff I.M. Acid-base indicators. (New York: The Macmillan Company, 1953).

23. Tobey S.W. The acid dissociation constant of methyl red. J. Chem. Educ. 1958. 35(10): 514. https://doi.org/10.1021/ed035p514

24. Schirmer R.E. Modern methods of pharmaceutical analysis. V. 1. (Boca Raton: CRC Press, 1982).

25. Tian Y., Wang G., Li F., Evans D.G. Synthesis and thermo-optical stability of methyl red intercalated Ni-Fe layered double hydroxide material. Mater. Lett. 2007. 61(8–9): 1662. https://doi.org/10.1016/j.matlet.2006.07.094