Anatomy and growth characteristics of conjugated polyelectrolyte/DNA aggregates

Abstract

Conjugated polyelectrolytes (CPEs) have been widely used as light harvesting macromolecules to amplify the signals of fluorescent assays that betray the presence of various biomolecular targets. Electrostatic interactions play an important role in coordinating optical coupling events and lead to the formation of complexes between oppositely charged CPEs and the target species. Here, we combine for the first time optical studies and structural characterization by liquid phase atomic force microscopy (AFM) to provide a picture of aggregate structure and growth dynamics between cationic CPE and DNA as a function of charge ratio. Specifically, Poly120, a copolymer containing a backbone with 50% fluorene, 30% phenylene, and 20% 2,1,3-benzothiadiazole (BT) units and pendant cationic groups was mixed with single stranded DNA (ssDNA) labeled with the Cy5 acceptor chromophore. Continuous addition of ssDNA-Cy5 to poly120 leads to a saturation in Cy5 emission due to Cy5-Cy5 self-quenching. Addition of ssDNA to a preformed poly120/ssDNA-Cy5 solution results in increased sensitization by energy transfer and a reduction of Cy5-Cy5 self quenching. Adsorption of aggregates onto negatively charged mica under water allows for direct imaging of the polyelectrolyte complexes as a function of charge ratio. The composite set of observations allows for the development of a model for aggregate growth, which is dynamic and ceases when the surface charge becomes sufficiently negative. This saturation point can be mitigated by addition of unlabeled ssDNA. © 2008 WILEY-VCH Verlag GmbH & Co. KGaA,.