Enhanced Polymeric Carbon Nitride Nanosheet-Based Fluorescence for Biosensing Applications

Two-dimensional (2D) nanomaterials have been extensively used as quenching platforms of fluorescently labeled oligonucleotides for the design of biosensors. However, not all fluorophore-labeled single-stranded DNA (ssDNA) is necessarily quenched after they are adsorbed on the surface of 2D nanomaterials. Herein, we discovered that the fluorescence of tetramethylrhodamine (TMR)-labeled ssDNA was not quenched but enhanced by polymeric carbon nitride nanosheets (PCN NSs). Effects of the fluorophore, oligonucleotide sequence, and the number of proximal G bases on the fluorescence of fluorophore-labeled ssDNA were investigated in the presence of PCN NSs. The fluorescence enhancement phenomena were not observable for fluorescein- and X-rhodamine-labeled ssDNA and other 2D nanomaterials such as graphene oxide and MoS2 NSs. In the presence of PCN NSs, steric hindrance effect shields the photoinduced electron transfer quenching effect between TMR and proximal G bases, which may account for the fluorescence enhancement of TMR-labeled ssDNA adsorbed on PCN NSs. Benefiting from the fluorescence enhancement effect and the affinity change of PCN NSs to ssDNA probes upon hybridization with complementary ssDNA, a versatile ratiometric biosensing platform as a proof concept was demonstrated for the high sensitive fluorescence detection of ssDNA and microRNA.