Temperature-jump and microspectroscopy technique for a study of mass transfer in a porous microparticle system

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We have developed fluorescence microspectroscopy techniques combined with laser temperature (T)-jump methods to analyzed chemical and physical processes for microparticles. Using a Nd^<3+>: YAG laser-Raman shifter, a light beam of 1200 nm (13 ns) was focused (spot size of〜20 μm) into water under an optical microscope. By the technique, T-jump greater than ΔT〜7K could be induced and fluorescence responses with temporal resolution of 10^8 s in the focused area were analyzed. Using a CW Nd^<3+>: YVO_4 laser, furthermore, another technique of T-jump greater than ΔT〜10 K (2〜3 μm) and temporal resolution of 1 ms was developed. These techniques were applied to phase transition and the microparticle formation by the phase separation of an aqueous poly (N-isopropylacrylamide) solution. The phase transition time could be directly determined to be 10 μs in solution. In 50 nm-sized pores of a microparticle, the phase transition time was greater than 10 ms. Intraparticle diffusion of coumarin 101 in ODS silica gel possessing pore diameter (d_p) of 12 nm could be measured by the CW laser T-jump technique. As comprehensive measurements, using single microparticle injection and absorption microspectroscopy, intraparticle diffusion was analyzed by quick concentration or pH change. Intraparticle diffusion in ODS silica gel with (d_p)=12 nm was limited by pore diffusion, analogous to that in silica gel with d_p=3〜30 nm. In a chitin derivative microsphere with d_p=〜1 nm, on the other hand, the rate-determining step of the intraparticle diffusion was surface diffusion. We conclude that the microscopic mechanisms of mass transfer in microparticle systems are successfully elucidated by potential applications of the present techniques.