Adsorption Of Na+ Ions On Sol-gel Films
ADSORPTION OF NA+ IONS ON SOL-GEL FILMS
IN OPTICAL FIBRE SENSORS
J. Rayss, G. Sudolski, A. Gorgol, W. Janusz, A. Gałgan
Faculty of Chemistry, Maria Curie-Skłodowska University, Maria Curie-Skłodowska sq. 3, 20-031 Lublin, Poland
e-mail: jrayss@hermes.umcs.lublin.pl
Unique features of sol-gel method have attracted attention of researches and gave pulse to broad research works in many fields of applications of sol-gel materials. In consequence these materials have found many application areas, e.g. antireflective and protecting coatings, mirrors, catalysis and many others. Along with the development of the optical fibre sensors sol-gel layers were applied to such a sensor as a matrix for entrapment of optical transducers, species, which change their optical properties (absorption, fluorescence etc.) when analyte concentration changes. Porous character of sol-gel glass enables easy access of analyte molecules into matrix and prevent optical transducer leakage. The sol-gel films were applied in intrinsic fibre optic sensor where the sol-gel film doped with optical transducer becomes integral part of the fibre core.
Despite the careful and very broad investigations being done, sol-gel materials are still in scope of continues interest. As to our knowledge, there are not in the literature the traces of investigations of dependencies between the sorption properties and optical behaviour of sol-gel layers in optical fibre sensors. In this paper we present the preliminary results of measurements of influence of sodium ions adsorption on value of response of pH optical fibre sensor with sol-gel film. We have found that the changes of a measured sensor absorbance is strongly associated with cations adsorption. We propose the following explanation of this phenomenon: in the region of low pH values up to 7 the surface charge of silica is negative and changes slowly with the pH increase. Hence, the adsorption of cations is also almost pH independent in this pH range. Contrary, for pH higher than 7, the negative surface charge of silica increases rapidly. As the consequence, increase of the cation adsorption is observed. Because the higher electron density of SiONa groups than SiOH,
the higher refractive index of sol-gel film. The increase of refractive index of sol-gel film causes the loss of light propagating through the optical fiber sensor. pH depending adsorption of cations in sol-gel film was independently confirmed by potentiometric titration and radiotracer technique.
Optical measurements and alternative methods give results being in good agreement.
The described above optical fibre sensor gives the possibility of rough estimation of pH and also porosity of sol-gel film when the time dependence of the optical signal is considered.
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