Two-Photon Absorption of Cationic Conjugated Polyelectrolytes: Effects of Aggregation and Application to 2-Photon-Sensitized Fluorescence from Green Fluorescent Protein

Two cationic conjugated polyelectrolytes (CPEs) based on poly(phenylene ethynylene) are found to exhibit efficient 2-photon excited fluorescence. In particular,. the CPE aggregates display enhanced 2-photon absorption compared to the nonaggregated states. The CPEs are used as 2-photon absorption sensitizers to harvest light and amplify the fluorescence from green fluorescent protein (GFP) through Forster resonance energy transfer. Considerably enhanced GFP fluorescence under near-infrared 2-photon excitation is demonstrated both in solution and in HeLa cells. The results suggest applications in 2-photon fluorescence imaging of cells, tissues, and organs of living animals expressing GFP.