Download OPTICAL pH DETECTION WITH U-SHAPED FIBER

OPTICAL pH DETECTION WITH U-SHAPED FIBER-OPTIC PROBES
AND ABSORPTION TRANSDUCERS
a
Jakub Zajíc, aLenka Traplová, bVlastimil Matějec, aMarie Pospíšilová, bIvo Bartoň
a
Czech Technical University in Prague, Faculty of Biomedical Engineering, ….
b
Institute of Photonics and Electronics AS CR, v.v.i, …
Measurements of pH are broadly employed in industry, agriculture, biology, medicine, etc.
In medicine the knowledge of pH value provides us with the information not only about the patient
status but also about some details on physiological processes in the patient body. In order to measure
pH different sensors including electrical and optical ones are used. Fiber-optic sensors have also
been tested for pH monitoring in medicine [1], e.g. for monitoring gastrohageal reflux. They can be
easily inserted in medical catheters. Usually, reflection-based fiber-optic probes are used in such
measurements which are modified by absorption or fluorescence pH indicators. Although such
probes are suitable for practical pH measurements the research and development of pH probes still
continues with aims such as suppressing the pH indicator leaching and decomposition or ways for
further miniaturization of sensing probes. This contribution presents results of the investigation of
optical properties of three absorption pH indicators, namely methyl orange, methyl red and
bromothymol blue. The indicators are immobilized in the cladding of U-shaped fiber-optic probes
by using matrices of porous silica, porous silica doped with titanium dioxide or ethylcellulose.
Three pH indicators, namely methyl orange (a pH detection range of 3.1-4.4), methyl red (a
pH range of 4.4-6.2) and bromothymol blue (a pH range from 6 to 7.6), have been investigated. In
order to develop pH sensors indicators were immobilized in layers prepared from alkoxides or from
polymer, ethylcellulose. Alkoxide-based layers were fabricated by the sol-gel method from sols
based tetrametylortosilicate and titanium tetrabutoxide dissolved in alcohols under the acidic
catalysis. Detection layers with thicknesses of about 0.5μm can be prepared from such matrices.
The ethylcellulose detection layers were applied from a sol of ethylcellulose in ethanol and toluene
with thicknesses of about 100μm. The sols were applied onto microscopic slides and U-shaped
fiber-optic probes by the dip-coating method. The applied coatings were dried at 100 °C. U-shaped
probes used in experiments were prepared from silica fibers (a core diameter of 400 μm) which
were bent in a burner flame to an arch with a diameter of about 2 mm. The arch dimensions were
fixed and the arch was coated with a sol or solution containing the tested pH indicators.
Transmission spectra of coated microscopic slides and U-shaped elements were measured
by using a halogen lamp as a light source and an Ocean Optics diode array spectrometer as a detector.
Slides and U-shaped elements were immersed in buffer solutions with different pH and changes of
absorption bands of the indicators were monitored. Temporal changes of measured spectra were
also measured. In the contribution spectra of the indicators and calibration curves in aqueous
solutions and in the matrices are discussed in terms of the indicator dissociation constant, the
detection sensitivity, linearity of the response, etc.
This work was financially supported by the project” BIO-OPT-XUV Research Team Advancement at the Faculty
of Biomedical Engineering, CTU in Prague, No.” MEYS ESF project CZ.1.07/2.3.00/20.0092”
[1] F. Baldini, Optical chemical sensors, Springer 2006