Fiber optic sensors based on hybrid phenyl-silica xerogel films to detect n-hexane: determination of the isosteric enthalpy of adsorption

Abstract

We investigated the response of three fiber optic sensing elements prepared at pH 10 from phenyltriethoxysilane (PhTEOS) and tetraethylsilane (TEOS) mixtures with 30, 40, and 50% PhTEOS in the silicon precursor mixture. The sensing elements are referred to as Ph30, Ph40 and Ph50, respectively. The films were synthesized by the sol–gel method and affixed to the end of optical fibers by the dip-coating technique. Fourier transform infrared spectroscopy, N2 adsorption–desorption at 77 K and X-ray diffraction analysis were used to characterize the xerogels. At a given pressure of n-hexane, the response of each sensing element decreased with temperature, indicating an exothermic process that confirmed the role of adsorption in the overall performance of the sensing elements. The isosteric adsorption enthalpies were obtained from the calibration curves at different temperatures. The magnitude of the isosteric enthalpy of n-hexane increased with the relative response and reached a plateau that stabilized at approximately −31 kJ mol−1 for Ph40 and Ph50 and at approximately −37 kJ mol−1 for Ph30. This indicates that the adsorbate–adsorbent interaction was dominant at lower relative pressure and condensation of the adsorbate on the mesopores was dominant at higher relative pressure.

Novel Organochlorinated Xerogels: From Microporous Materials to Ordered Domains

“…Organically modified silicates (ORMOSILs) are hybrid silicon xerogels that combine the mechanical, thermal, and structural rigidity and stability of inorganic materials with the flexibility and functionality of organic molecules [ 1 ]. These properties favor their utilization as chemical and optical sensors [ 2 , 3 , 4 , 5 , 6 , 7 ], catalysts [ 8 , 9 ], coatings [ 10 , 11 , 12 , 13 , 14 ], chromatographic agents [ 15 , 16 ], nanoprobes, and photovoltaic cells [ 17 , 18 ], among others.…”

Micro and Nanostructured Materials for the Development of Optical Fibre Sensors

“…In the case of silica based sol gels, the final matrix can act as the sensing element itself with no need of a sensing material. It has been hypothesized that silanol groups might interact with molecules showing lone pairs of electrons, which is the case of some VOCs: as a result, the optical properties of the coating changes and variations in the absorbance spectra are used to detect the VOCs presence [ 82 ]; even relative humidity sensors are reported employing just xerogel as the sensing element (see Figure 20 ) [ 83 ]. Sol gels have been also coated onto optical devices such as stretched fibres and micro couplers: the response of this sensors is based on spectral shifts.…”

Hybrid Xerogels: Study of the Sol-Gel Process and Local Structure by Vibrational Spectroscopy

“…For this reason, it is important to prepare materials with different porosities and surface chemistries in which the interaction between the membrane (chemical area of a sensor) and the analyte is specific and labile. To date, our research group has prepared membranes with hybrid xerogels obtained using different molar ratios of TEOS:RTEOS (R = methyl or phenyl) [ 33 , 34 ]. Given the results obtained and following this line of reasoning, the addition of a chlorine atom to the organic moiety of a silane emerged as a highly appealing approach because of the inductive effects of the chlorine, which generates a more active chemical surface and favors functionalization with other compounds.…”

“…Physisorption of the analyte on the surface of the material generates a modification of the refractive index and produces a change in the reflected optical power, which determines the analyte concentration in the medium. This study is of crucial importance because: (i) precise knowledge of the gelation time is essential to effectively impregnate the fibers [ 33 , 34 ], and (ii) complete characterization of the xerogels allows prediction, to a large extent, of their properties and the a priori selection of the xerogel best suited to the characteristics of the analyte of interest [ 32 ].…”