Interpolyelectrolyte complexes of anionic water-soluble conjugated polymers and proteins as platforms for multicolor protein sensing and quantification

A simple method for protein detection and quantification has been developed by taking advantage of the aggregation-induced fluorescence change of anionic water-soluble conjugated polymers. These polymers contain charged carboxylate groups and a pi-electron delocalized optically active backbone composed of fluorene segments and 2,1,3-benzothiadiazole (BT) units. The polymers were synthesized through the Suzuki coupling between 2,7-[bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-bis(3-(tert-butyl propanoate))]fluorene, 2,7-dibromo-9,9-bis(3'-(tert-butylpropanoate))fluorene, and 4,7-dibromo-2,1,3-benzothiadiazole, which was followed by treatment in trifluoroacetic acid to afford the functional carboxylic acid groups. P1-BTx and P2-BTx refer to the neutral precursor polymers and the anionic water-soluble polymers, respectively. The subscript in P1-BTx and P2-BTx (x = 7.5, 15, 30) refers to the molar percentage of BT units in the polymer backbone, which is 7.5%, 15%, and 30%, respectively. Both the optical spectra and the light scattering studies show that the polymers are aggregated in water at low pH, and the aggregation decreases at high pH. Along with the aggregation and aggregation breakup processes, the polymer emission also changes from yellow to blue in solution. At pH > 9, where most carboxylic acid groups are deprotonated, intense blue fluorescence is observed for all three polymer solutions. Using P2-BT30 as an example, addition of proteins to the polymer solution results in a change of emission color from blue to yellow, green, and dark for lysozyme, bovine serum albumin, and cytochrome c, respectively. The color change is due to efficient intramolecular/intermolecular energy transfer from the fluorene segments to the BT sites or electron transfer between the polymer and proteins upon complex formation. The variation in polymer emission color in the presence of different proteins is due to the difference in hydrophobic nature, net charge, and the structure among proteins. As demonstrated with P2-BT30 and lysozyme, the protein-induced polymer emission change has also been used to quantify protein concentrations.