Characterization of the microenvironments of PRODAN entrapped in tetraethyl orthosilicate derived glasses

6-Propionyl-2-(dimethylamino)naphthalene (PRODAN) has been widely used to p robe the internal environment of sol-gel derived glasses. It is generally a ssumed that the entrapped probe reports primarily on the internal solvent e nvironment, through changes in emission wavelength, lifetime, or anisotropy . However, we show that other effects, such as aggregation of the probe and adsorption of the probe onto the silica surface, can also alter the emissi on properties of PRODAN, providing further information on the evolution of sol-gel derived classes. Both the steady-state and time-resolved fluorescen ce properties of PRODAN were examined when the probe was entrapped in tetra ethyl orthosilicate (TEOS) derived classes. The classes were prepared using a two-step method that is commonly used for protein entrapment, and aged e ither in air without washing (dry-aged), in air after a washing step (washe d), or in buffer (wet-aged). For all aging methods, the changes in the emis sion properties of the probe were consistent with at least three discrete m icroenvironments, reflecting free monomers, free aggregates, and adsorbed s pecies (monomers and/or aggregates), the proportion of which changed as a f unction of drying time and conditions. The monomeric form of the probe unde rwent a polarity-sensitive emission shift that reflected changes in the int ernal solvent composition. However, the aggregates/adsorbates contributed u nique features to both the steady-state and time-resolved emission properti es of PRODAN that gave insights into changes in the solubility of the probe , consistent with loss of internal solvent as aging of the glass proceeded. This study clearly shows that significant new information can be obtained from studies of PRODAN emission, and demonstrates that time-resolved fluore scence measurements are critical to properly elucidate the environment(s) p resent within the sol-gel derived materials.