Preparation and characterisation of calcein-doped thin coatings

Silica sol-gel coatings doped with calcein (CA) were prepared and optically charecterised by means of UV-VIS spectroscopies (optical absorption and luminescence). In addition, the stability of samples against thermal treatment, UV exposure and alkali attack (chemical durability) were studied. Microstructure was verified using transmission electron microscopy and specific surface area was measured using the BET method. Results highlighted the ability of sol-gel coatings to react reversibly to pH changes when immersed in liquid media at different pH values. The limits for maximum optical absorption sensitivity were determined to be 5 < pH < 9, while the 8.5 < pH < 14.0 and 0 < pH < 2 ranges could be detected using photoluminescence spectroscopy. Stability studies showed good behaviour against UV (maximum loss of 50% in peak intensity after 30 h of continuous exposure) and good chemical durability under alkaline conditions (80 h attack time was necessary to reduce the percentage absorption to 50%). Thermal stability of doped films was good up to 130 degreesC (in this range a reduction of about 25% in the intensity of the initial peak absorption is lost).