Label-Free DNA Detection through Energy Transfer of Conjugated Polymer Complexes

Label-free DNA detection is a highly sensitive and selective method with great scientific and social significance. Water-soluble cationic conjugated polymer (CCP), poly(9,9-bis(6'-N,N,N-trimethylammonium)-hexyl)-fluorene phenylene) (PFP), was synthesized by cross coupling reaction under the catalysis of palladium, and water-soluble anionic conjugated polymer (ACP), poly(3-thiophene sodium acetate) (P3TSA), was prepared through oxidative addition polymerization with anhydrous ferric chloride as the catalyst. Both PFP and P3TSA were well characterized by H nuclear magnetic resonance (H NMR) and Fourier transform infrared (FTIR) tests. Owing to the existence of opposite ionic side-chains, PFP and P3TSA were both water-soluble and could form polymer complexes through electrostatic interactions. The transmittance measurements of the polymer complex solution with wavelength ranging from 600 to 750 nm were performed. The results showed that the transmittance was almost unchanged for all samples, confirming the high solubility and stability of polymer complexes. The ultraviolet-visible (UV-vis) and photoluminescence emission measurements showed that the emission spectrum of PFP overlapped a lot with the absorption spectrum of P3TSA, proving the energy transfer from PFP (donor) to P3TSA (acceptor). By fixing the donor concentration, energy transfer efficiency (ETEF) of the polymer complexes increased with the concentration of P3TSA. With the enhancement of P3TSA concentration, the ETEF increased slowly at first and then relatively fast at a high P3TSA concentration. A sample with a high ETEF was selected to investigate the effect of DNA probe (X1) concentration on ETEF. It was observed that ETEF decreased with the increase of X1 concentration, owing to the repulsion of P3TSA by negative charged DNA probes. DNA hybridization was conducted by using 0.2 nmol of X1 as a probe. The experimental results showed that the perfectly matched, double mismatched and non-matched target DNA could be well distinguished with obvious photoluminescence signals. In conclusion, a new label-free DNA detection method with high selectivity has been successfully developed based on the energy transfer of conjugated polymer complexes.