The talk will focus on the photophysics of fluorescent conjugated polyelectrolytes and their very high sensitivity (superquenching) to quenching by small molecules that can associate with the polymers and interact via energy or electron transfer. Superquenching occurs for the polymers in solution and also when they self-assemble on microspheres or nanoparticles. The application of superquenching to biosensing has provided a means for high sensitivity detection of enzyme activity, proteins and nucleic acid hybridization. It has also been found that these polymers have biocidal activity towards bacteria and bacterial spores. The origin of the light-induced biocidal activity will be discussed.