High-pressure total internal reflection fluorescence apparatus

Important interfacial processes in disciplines ranging from medicine to the separations sciences occur over a wide range of pressures, temperatures, and time scales. In this paper we report a new highpressure total internal reflection fluorescence (HP-TIRF) apparatus that allows rapid fluorescence measurements of sub-monolayers in contact with liquids and supercritical fluids between 293 K and 353 K and up to 250 bar with picosecond time resolution. We use the HP-TIRF system to study the in-plane rotational reorientation dynamics of the fluorescent probe BODIPY 494/503 (C_2v symmetry) covalently attached to silica surfaces that have been silanized with n- propyltrimethoxysilane (C₃-TMOS) or 3,3,3- trifluoropropyltrimethoxysilane (CF₃-TMOS) when the interface is subjected to pure supercritical carbon dioxide (scCO₂). The in-plane BODIPY 494/503 rotational reorientation dynamics are assessed by using the Debye-Stokes-Einstein expression. As the scCO₂ density increases the local microviscosity surrounding the tethered BODIPY 494/503 molecule decreases. The terminal group (CH₃ versus CF₃) within the silane monolayer governs the onset and absolute magnitude of the observed viscosity changes. The results are explained in terms of the wellknown solubility of fluorine-containing species in scCO₂.