Upconversion Fluorescence Resonance Energy Transfer Biosensor with Aromatic Polymer Nanospheres as the Lable-Free Energy Acceptor

We report a new upconversion fluorescence resonance energy transfer (UC-FRET) biosensor using poly-m-phenylenediamine (PMPD) nanospheres as the energy acceptor in this paper. A single stranded DNA (ssDNA) tagged with a sulfydryl group at the 5'-terminus was covalently linked to poly(ethylenimine) (PEI) functionalized upconversion phosphors (UCPs, the energy donor). Because of the pi-rich electronic structure of PMPD, self-assembly of the donor and the acceptor was achieved through the pi-pi stacking interaction between ssDNA and PMPD. The fluorescence of the donor was quenched by the acceptor in a PMPD-concentration-dependent manner. A maximum quenching degree of 90% was acquired, which was among the highest levels of all previous reports. Upon the formation of double-stranded DNA (dsDNA) between the target DNA and the probe DNA, the energy acceptor was separated from the donor due to the weakened interaction between dsDNA and PMPD. The fluorescence of UCPs was accordingly restored, and a linear response was obtained with the target concentration ranging from 0.1 to 6.0 nM. The limit of detection was calculated as 0.036 nM, which was a highly competitive sensitivity. The sensor also showed high precision, pronounced specificity, and the applicability to complicated sample matrix (human serum). The UCPs-PMPD FRET sensing platform takes advantages of both the optical merits of the upconversion donors and the superquenching ability and good water-solubility of the aromatic polymer nanoparticles. This study will open the opportunity to develop a new class of UC-FRET biosensors.