Abstract: A method of optically characterizing individual molecules/molecular complexes, or other particles, in solution. The method comprises flowing a solution comprising the molecules/molecular complexes into an imaging region of a microfluidic channel, wherein the imaging region of the microfluidic channel has a first lateral dimension of greater than 1 ?m in an x-direction wherein the x-direction is perpendicular to a direction of the flow; capturing a succession of images of the individual molecules/molecular complexes in the imaging region; tracking movement of the individual molecules/molecular complexes in at least the x-direction in the imaging region using the succession of images; and characterizing the individual molecules/molecular complexes from the tracked movement. In some implementations the characterizing comprises determining a diffusion coefficient of the molecules/molecular complexes from the tracked movement.