Single-microparticle measurements: Laser trapping-absorption microspectroscopy under solution flow conditions

A laser trapping-microspectroscopy system combined with a fluid manifold was developed to manipulate and analyze "single" microparticles, A sample solution containing microparticles was introduced to a now cell set on a microscope stage, and a single particle was trapped by a 1064-nm laser beam. With the particle being trapped, the other particles were pumped out by flowing water to hold the unique microparticle in the now cell. Under solution-flow conditions, a single microparticle was laser trapped in balance with the gradient (F-g) and Stokes forces (F-s) experienced by the particle, and thus, the trapped position was shifted to the downstream side of the 1064-nm laser beam focus. mow rate and particle size dependencies of this particular positional displacement of the particle were discussed in terms of F-g and F-s. On the basis of these studies, optical requirements to conduct absorption microspectroscopy of a laser-trapped particle were optimized, and the technique was applied to study a time course of dye adsorption processes in single microparticles, The adsorption rate of Rhodamine B was determined for individual microparticles for the first time.