Spatially distributed optical excitation and integrated waveguides are used for ultrasensitive particle detection based on individual electrokinetic velocities of particles. In some embodiments, chip-integrated systems are used to identify individual particles (e.g., individual molecules) based on their velocity as they move through an optically interrogated channel. Molecular species may be identified and quantified in a fully integrated setting, allowing for particle analysis including molecular analysis that can operate at low copy numbers down to the level of single-cell lysates. In some embodiments, the single-particle velocimetry-based identification and/or separation techniques are applied to various diagnostic assays, including nucleic acids, metabolites, macromolecules, organelles, cell, synthetic markers, small molecules, organic polymers, hormones, peptides, antibodies, lipids, carbohydrates, inorganic and organic microparticles and nanoparticles, whole viruses, and any combination thereof.