Translational diffusion of probe molecules under high pressure: A study by fluorescence recovery after photobleaching technique

Abstract

International audienceWe present fluorescence recovery measurements after photobleaching performed under high pressure in liquids that fill square-section fused silica micro-capillaries. These micro-capillaries withstand pressure up to 2500 bar for a wall thickness of about 140 μm and fit easily on the microscope stage. This technique allows the translational diffusion coefficient of fluorescent molecules in liquids to be measured as a function of pressure. When the liquid sample is far from its glass transition the translational diffusive coefficient is in agreement with the Stokes-Einstein equation. As the glass transition is approached by further increasing the pressure, decoupling of the measured diffusion coefficient from the Stokes-Einstein relation is observed. These are the first measurements that combine the fluorescence recovery technique and high hydrostatic pressures. This experimental setup can also be used either with diamond or sapphire anvil cells in order to span a larger pressure range

Pressure-Induced Glass Transition Probed via the Mobility of Coumarin 1 Fluorescent Molecule

“…Along the isotherm at T = 30 °C their measurements show that the shear viscosity varies exponentially with pressure with an ∼2.5 orders of magnitude increase between atmospheric pressure and 0.4 GPa. Because of the lack of high-pressure viscosity measurements for PPG-1000M, we assume that the viscosities of PPG and PPO obey the same pressure dependence: where μ 0 ( P = 0.1 MPa) is the shear viscosity depending on the polymer and c = (6.2 ± 0.4) × 10 –3 MPa –1 is constant for both PPG and PPO. For PPG-1000 M the value of μ 0 ( T ) is taken from Baur et al Figure shows that up to ∼0.3 GPa the normalized diffusion coefficient D / D SE is centered around 1.…”

“…The recovered spot fluorescence intensity is the spatially integrated product of the intensity profile of the reading beam with the concentration distribution of unbleached molecules that diffuse into the bleached area. For 2-D diffusion many analytical expressions have been derived to fit the recovered time-dependent fluorescence intensity and deduce the diffusion coefficient of the fluorescent molecules. ,, In a previous article, we have shown that a single-exponential function can fit the data on a time scale as large as 10 times the diffusive time. Van Keuren and Schrof derived a simple analytical expression for the fluorescence recovery in a small region centered on the bleached spot.…”

“…In this paper we investigate the mobility of a fluorescent molecule in different solvents as a function of pressure up to ∼6 GPa. This research is a continuation of high-pressure fluorescence recovery after photobleaching (FRAP) measurements in poly­(propylene glycol) employing square-section quartz microcapillaries that could sustain relatively low pressures up to ∼300 MPa . Tracer mobility is connected to the translational diffusion coefficient of the probe molecules that interact with the host sample and can be related to the S–E equation.…”

“…The tracer diffusion is measured by FRAP. This technique allows very long diffusive times to be recorded and is thus particularly adapted for systems approaching a glass transition . High pressure is applied by means of a diamond anvil cell (DAC cell), utilizing ultralow fluorescent UV-transparent diamond.…”

“…This research is a continuation of high-pressure fluorescence recovery after photobleaching 34 (FRAP) measurements in poly(propylene glycol) employing square-section quartz microcapillaries that could sustain relatively low pressures up to ∼300 MPa. 35 Tracer mobility is connected to the translational diffusion coefficient of the probe molecules that interact with the host sample and can be related to the S− E equation. Pressure may alter the structural architecture of the host liquid, hence the coupling between the probe molecules and the solvent.…”