2. Materials and methods
Toluidine Blue (TB) was obtained from ALDRICH. Mono- and di-basic potassium phosphate (Cicarelli) for buffer solutions were used as received. PH value was adjusted with hydrochloric acid or potassium hydroxide. Titanium (IV) oxides, anatase (25 nm particle size) and rutile nanopowders (100 nm particle size) were obtained from ALDRICH. Ultrapure water (18 MΩ.cm) was obtained using a Milli-Q water purification system (Millipore).
To fabricate the sensor, plastic optical fibers were used. This kind of fiber is commercially available as standardized optical fiber connection systems (Toslink). The fiber is composed by a 920 μm OD polymethylmetacrilate (PMMA) core, a fluorinated polymer clad with a thickness of 20 μm and a black polyethylene jacket. The core and clad refraction indexes are 1.49 and 1.46 respectively.
A ten centimeters piece of plastic fiber was cut and the black jacket was completely removed, the middle region of the fiber was carefully decladded to partially expose the core. The entire piece was bent slowly until it became U-shape with the de-cladded zone in the curved area, and immobilized on a polyamide plug by introducing the extremes in two positioned holes performed for this purpose. A white LED acts as light source at one end of the fiber while the output signal is directed to an optical fiber (Ocean Optics Inc.) coupled to a HR2000 Ocean Optics Spectrometer (Ocean Optics Inc.) connected to a laptop. A quartz beaker compatible in size with the fabricated polyamide plug was used to put the sensor in contact with 10 mM pH: 7.0 phosphate buffer solution. Several aliquots of 2.5 × 10−3 M TB standard solution were added to the buffer medium and the optical fiber attenuation was registered (computed as an absorbance change). The measured signal is produced by the interaction between adsorbed Toluidine Blue on the fiber surface and the evanescent wave in the interface fiber/solution. All the experimental work has been done with magnetic stirring at room temperature.
Adsorption of Toluidine Blue on Titanium IV oxide was used to evaluate the sensor performance in a high turbidity medium. A known quantity of TiO2 was dispersed in a small volume of phosphate buffer or Toluidine Blue solutions taken from the experimental system using a Pasteur pipet and then the dispersion was returned immediately to the system. A Shimatzu P600 UV-Visible spectrophotometer was used to contrast the obtained results.
Photodegradation of Toluidine Blue adsorbed on TiO2 with different crystal phases was tested using the same optochemical sensor. For this purpose, TB adsorbed on TiO2 was irradiated with a 300 W Osram Ultra Vitalux mercury tungsten blended UV reflector.
The sensor was cleaned and reused after every experiment. The cleaning process consists of a washing step with buffer solution during 20-30 minutes under convection, and then the fiber is gently cleaned with a wet tissue paper. The fiber was always stored into 10 mM pH= 7.0 phosphate buffer solution.