Solvent-assisted optical modulation of FRET-induced fluorescence for efficient conjugated polymer-based DNA detectionElectronic supplementary information (ESI) available: Additional UV-vis, emission, FRET-induced PL spectra, and time-resolved spectroscopy data in PBS/DMSO mixtures. See DOI: 10.1039/c0cp01025a

The solvent effects were studied in fluorescence resonance energy transfer (FRET) from a cationic polyfluorene copolymer ( FHQ , FPQ ) to a fluorescein (Fl)-labelled oligonucleotide (ssDNA-Fl). Upon addition of dimethyl sulfoxide (DMSO), the optical properties of polymers and the probe dye were substantially modified and the FRET-induced PL signal was enhanced 3.8-37 times, relative to that in phosphate buffer solution (PBS). The hydrophobic interaction between polymers and ssDNA-Fl is expected to decrease in the presence of DMSO, which induces the weaker polymer/ssDNA-Fl complexation with longer intermolecular donor-acceptor separation and perturbs the competition between the FRET and PL quenching processes such as photo-induced charge transfer. The gradual decrease in Fl PL quenching with increasing the DMSO content was investigated by measuring the Stern-Volmer quenching constants (3.3-4.2 × 10 6 M −1 in PBS, 0.56-1.1 × 10 6 M −1 in 80 vol% DMSO) and PL lifetime of the excited Fl* in polymer/ssDNA-Fl (600 ps in PBS and 2120 ps in 80 vol% DMSO for FHQ /ssDNA-Fl) in PBS/DMSO mixtures. The substantially reduced PL quenching would amplify the resulting FRET Fl signal. The signal amplification in real DNA detection was also demonstrated with fluorescein-labelled PNA (probe PNA) in the presence of a complementary target DNA and noncomplementary DNA in aqueous DMSO solutions. This approach suggests a simple way of modifying the fine-structure of polymer/ssDNA-Fl and improving the detection sensitivity in conjugated polymer-based FRET bioassays. The FRET-induced fluorescence was substantially enhanced (3.8-37 times) upon addition of dimethyl sulfoxide in conjugated polymer-based FRET DNA detection in aq. buffer solution.