The application of laser radiation as a method for the manipulation of microscopic particle suspensions for biological, thermodynamic and microfluidic interests has brought about a revolution in micro-scale research across many different scientific disciplines. It has been shown that a diffraction limited focused laser can be used to trap microscopic particles whose refractive index is greater than their surrounding solvent. Termed Optical Trapping, work in this arena has yielded, new methods, techniques and applications that have flourished, and applications of this technology to areas of research involving microscopic systems for analysis, detection, separation and concentration have blossomed.
A related technique, Optical Chromatography, used for particle separation involves loosely focusing a laser into a fluid flowing opposite to the direction of laser propagation. When microscopic particles in the flow path encounter this beam they are trapped axially along the beam and are pushed upstream from the laser focal point to rest at a point where the optical and fluid forces on the particle balance. Because optical and fluid forces are sensitive to differences in the physical and chemical properties of a particle, fine separations are possible.
Recently, this method has been used to separate spores of different Bacillus species based on their optical and fluidic properties. We will describe how an optical chromatography beam directed into a tailored flow environment housed in a glass flowcell, has been adapted to operate as an optically tunable filter for the concentration or bioenrichment of colloidal and biological samples. Application of these methods and further design of fluidic and optical environments will allow for more specific identification, concentration and separation of many more microscopic particle and biological suspensions.