Provided herein are new methods and apparatus for quantitative measurement and analysis of particles, including new apparatus systems to process and detect nanoparticles in suspension. By focusing a laser beam at the center of a reservoir, nanoparticles are concentrated by optical energy, and fluorescent intensity at the focal point of the laser is measured to quantify particle concentration in the reservoir. The techniques may be applied to the analysis of suspensions of nanoparticles, including natural particles (e.g., microorganisms including whole viruses, bacteria, animal cells, and proteins) and synthetic particles (e.g., colloidal latexes, paints, pigments, and metallic or semiconductor nanoparticles) for medical and industrial applications, among others.