Photometric analyses of optically-trapped single microparticles in solution

We explored optical trapping-spectrophotometry of ion-exchange polymer microparticles in water and, determined the dye (tris(2,2'-bipyridine)ruthenium(II) complex; Ru2+) concentration adsorbed on an individual particle (diameter (d)=20 - 140 mu m). The absorbance of Ru2+ on the particle increased with an increase of the dye concentration in the aqueous phase, while it was essentially independent of d. The results indicate that adsorption of the complex takes place in the surface layer of the particle, but not homogeneously in the entire particle. A possible role of the optical trapping-spectrophotometry method in direct analyses of trace amounts of molecules concentrated on single microparticles in solution is discussed.