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 lu minescence). In addition, the stability of samples against thermal treatmen t, UV exposure and alkali attack (chemical durability) were studied. Micros tructure was verified using transmission electron microscopy and specific s urface area was measured using the BET method. Results highlighted the abil ity of sol-gel coatings to react reversibly to pH changes when immersed in liquid media at different pH values. The limits for maximum optical absorpt ion 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 spectrosco py. Stability studies showed good behaviour against UV (maximum loss of 50% in peak intensity after 30 h of continuous exposure) and good chemical dur ability under alkaline conditions (80 h attack time was necessary to reduce the percentage absorption to 50%). Thermal stability of doped films was go od up to 130 degreesC (in this range a reduction of about 25% in the intens ity of the initial peak absorption is lost).