Optical Fibre Ph Sensor Based On Ion Adsorption
OPTICAL FIBRE pH SENSOR BASED ON ION ADSORPTION
IN SOL-GEL LAYER
G. Sudolski, J. Rayss
Laboratory of Optical Fibre Technology, Faculty of Chemistry,
Maria Curie-Sklodowska University,
Maria Curie-Sklodowska sq. 3, 20-031 Lublin, Poland
e-mail: sudolski@hermes.umcs.lublin.pl
In this paper an optical pH sensor based on ion adsorption is presented. As optical transducer porous layer of silica, which was prepared by means of sol-gel method, was utilised. There was no pH sensitive indicator incorporated in the sol-gel layer.
It is well known that absorption of cations on porous forms of silica is strongly pH dependent due to chemistry of silanol groups presented on the silica surface. The surface charge appearing thanks to ionisation of silanol groups gives pulse to cations adsorption. Moreover adsorbed ions increase electron density of the film considered as a bulk material. This, in turn, manifests with increase of refractive index of the film that can be optically detected.
A sensing section of the fibre sensor (optrode) was produced by replacement of an original fibre cladding with the sol-gel film. In such a sensor system variations of the refractive index of the film, caused by adsorption of cations being consequence of pH change affects strongly propagation conditions of the light carried within the fibre. In this way pH variations can be optically detected.
The length of the sensing section was about 10 cm. In order to miniaturise the sensing section of the sensor optrode was U-shaped. As a silica precursor tetraethoxysilane was used. The sol-gel cocktail was prepared to hold constant the tetraethoksysilane:acidifiedwater(pH=2):ethanol ratio equals 1:4:4.
PCS (Plastic Clad Silica) fibre deprived original cladding was pulled trough the sol mixture at 1 mm/s speed. The fibres with films cladded on core gelled either in ammonia or ambient atmosphere were compared.
It was found that the sensing region was 7.5-10.5 pH with sensitivity higher than 0.04 signal unit (absorbance) per pH unit, which allows pH evaluation with accuracy better than 0.25 pH unit. Better linearity of the sensor response vs. pH was observed for ammonia treated optrodes, but better signal stability and repeatability were observed for oprtodes without such treatment.
There was no signal difference in sensor characteristic for samples containing KOH and NaOH.
The response time (evaluated as a time when 90 % of the total signal was reached) were up to 1 min for ammonia treated optrodes and from 2 to 7min for the fibres gelled in room atmosphere.
Ionic strength influence on sensor response was also investigated. As it was expected the increase of ionic strength causes increase of sensor signal at constant pH. However, this phenomenon is moderate in magnitude and can induce significant measurement errors for salts concentration exceeding 0.5mol/dm3.
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