Enhancement of Quantum Dot Förster Resonance Energy Transfer within Paper Matrices and Application to Proteolytic Assays

Brightly luminescent semiconductor quantum dots (QDs) continue to play an increasing role in biophotonic research and applications such as bioassays. Here, we present methods for the immobilization of QDs on the cellulose fibers of paper substrates for Förster resonance energy transfer (FRET)-based assays of proteolytic activity. Steady-state and time-resolved fluorescence characterization of FRET between immobilized QDs and self-assembled dye-labeled peptides within the paper matrix revealed a substantial enhancement in energy transfer efficiency. Compared to bulk solution, the rate of energy transfer increased approximately fourfold resulting in a concomitant sevenfold increase in the ratio of FRET-sensitized acceptor dye emission and quenched QD emission. Spots of immobilized QDs with different amounts of dye-labeled peptide had bright luminescence under UV/violet illumination and the net QD and Alexa Fluor 555 (A555) dye emission was visible by eye as different colors. Tryptic digestion of the peptides linking the QD donor and the acceptor dyes resulted in loss of FRET. Changes in the dye/QD photoluminescence (PL) ratio permitted the tracking of proteolytic activity, including the effect of increasing amounts of aprotinin, a potent inhibitor of trypsin. The combination of QDs, a paper substrate, and enhanced FRET has strong potential for developing bioassays.