Conjugated cationic polymer-assisted amplified fluorescent biosensor for protein detection via terminal protection of small molecule-linked DNA and graphene oxide

A sensitive and selective protein biosensor was proposed based on cationic conjugated polymer (PFP)-induced signal amplification with the aid of terminal protection of small molecule-linked DNA (TPSMLD) and graphene oxide (GO). The DNA probe labeled with fluorescein (FAM, P2) had a complementary sequence to the small molecule modified single-stranded DNA (P1). Taking streptavidin (SA) as the target protein, P1 labeled with biotin was digested to small nucleotide fragments by exonuclease I (Exo I) without SA. Upon adding PFP to P2-GO complex, weak fluorescence resonance energy transfer (FRET) from PFP to FAM-induced fluorescence signal was observed owing to the equal affinity for P2 and PFP toward GO. In the presence of SA, P1 was protected from the digestion of Exo I due to the specific binding between SA and biotin, releasing P2 from GO surface by hybridization to form double-stranded DNA (dsDNA). In this case, the addition of PFP led to obvious FRET signal because of strong electrostatic interactions between dsDNA and PFP. By monitoring the change of FRET signal, SA could be quantified. Compared to the strategy without PFP, the introduction of PFP showed an enhanced sensitivity with a detection limit of 1.6 ng mL−1 for SA detection. Also, folate reporter was detected based on this platform with a good linear relationship, demonstrating the generality of this design.