Determination of the Network Structure of Sensor Materials Prepared by Three Different Sol-Gel Routes Using Fourier Transform Infrared Spectroscopy (FT-IR)

Abstract

Solid acid-base sensor materials were prepared by encapsulating three pH indicators (alizarin red, brilliant yellow, and acridine) within a silica matrix using a sol-gel approach through three different routes: (1) non-hydrolytic, (2) acid-catalyzed, and (3) base-catalyzed. Raman and Fourier transform infrared spectroscopies were used to evaluate the silica-indicator interactions. Because vibrational bands assigned to functional groups present in the indicator molecules were not detected, the main silica stretching mode νSi-O between approximately 1300 and 1000 cm(-1) was used to detect the presence of our indicators within the silica matrix. The large band centered at 1100 cm(-1) was deconvoluted into four components corresponding to the longitudinal optic and transversal optic modes of the silicon monoxide (SiO)4 and (SiO)6 siloxane rings. Using the component area of each mode, it was possible to calculate the percentage of each structure. Such percentages ranged from 49% to 70% (SiO)6 for the analyzed samples, within a confidence level of 95% (p = 0.05). (The confidence limits were 53-62%.) These results could be related to the pH indicator content, indicating that the quantity of the encapsulated molecule affects the (SiO)6 percentage values. In addition, a comparison with the radius of gyration obtained by small angle X-ray scattering was done. These results indicate that the analyte accesses the receptor elements through the passages between the siloxane rings but not through the siloxane rings themselves.

Fourier Transform Infrared and Raman Characterization of Silica-Based Materials

“…In this case, the silica network itself limits the pathways the analyte can travel to reach the encapsulated receptor molecules hindering the reaction. Consequently, both cases could reduce the sensor performance or completely disable it [36].…”

“…Using this approach, a series of silica-based acid-base optical sensors prepared by encapsulating pH indicators using three different sol-gel routes was investigated [36]. The employed routes were: nonhydrolytic, acid-catalyzed and base-catalyzed and the pH indicators were alizarin red, brilliant yellow and acridine.…”

“…In addition, no relationship between the (SiO) 6 percentage and the response time could be established in spite of less dense networks, with bigger rings, might render easier the analyte permeation. This behavior may indicate that the analyte probably accessed the receptor elements through the passages between the siloxane rings and not through the siloxane rings themselves [36]. It is remarkable that depending on the sol-gel route, the rate between hydrolysis and condensation reactions will change with the pH medium and can explain the behavior of the basiccatalyzed sensor material.…”

“…The silica spectrum presents intense bands in the region of ~3500 cm , where the Si-O stretchings are observed. As a result of these strong bands and the frequently low concentrations of the encapsulated compounds, it is common to observe an overlapping of their signals by the silica ones avoiding this type of evaluation [36].…”

Correlating the Morphological Properties and Structural Organization of Monodisperse Spherical Silica Nanoparticles Grown on a Commercial Silica Surface

“…62, 67 The shoulders of the band centered at $\tilde \nu $ =3434 cm −1 are also due to vibrations of the OH groups. The shoulder at approximately $\tilde \nu $ =3644 cm −1 is characteristic of primary alcohols68, 69 and may be a result of residual ethanol from alcohol condensation during the synthesis or SiOH absorption, whereas the shoulder at approximately $\tilde \nu $ =3245 cm −1 is probably due to the OH H(⋅⋅⋅O) groups, wherein the OH groups interact through hydrogen bonds.…”