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 flow 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 flow 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. Flow 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.