Described are conjugated polymers and conjugated polymer nanoparticles formed therefrom. The conjugated polymers and conjugated polymer nanoparticles have a maximum emission of light that occurs within a tissue transparent window of the electromagnetic spectrum. These emission properties are particle-size independent. The sizes of the conjugated nanoparticles are controlled by altering the concentration of the conjugated polymer used to make conjugated polymer nanoparticles. Also described are methods of making conjugated polymer nanoparticles that have larger sizes than have been traditionally reported, involving a modified reprecipitation approach. The conjugated polymers and/or conjugated polymer nanoparticles can be used as fluorescent probes in biological imaging.