Förster Resonance Energy Transfer from Quantum Dots to Rhodamine B As Mediated by a Cationic Surfactant: A Thermodynamic Perspective

Förster resonance energy transfer (FRET) has attracted much attention for its wide applications in the fields of bioimaging, bioanalysis, etc. One of the critical problems in FRET is the construction of suitable donor–acceptor pair. The fluorescent quantum dots (QDs) can well meet the requirements both for a donor and an acceptor, owing to their tunable emission and broad absorption. Besides, the QDs possess high quantum yield, which highly benefits the FRET efficiency. In this work, glutathione-capped CdTe QDs (GSH-CdTe QDs) was chosen as the energy donor (D) and Rhodamine B (RhB) as the energy acceptor (A). However, no FRET occurred when there were only QDs and RhB, even though there was much overlap between the absorption spectrum of RhB and the emission spectrum of QDs. Interestingly, after the addition of a cationic surfactant, cetyltrimethyl­ammonium bromide (CTAB), FRET was induced favorably. Further understanding of this phenomenon was studied by fluorescence spectroscopy, dynamic light scattering, and zeta potential. The results indicated that QDs aggregated and were cross-linked by CTAB due to electrostatic interactions. Then, RhB was trapped in the aggregates. Therefore, QDs and RhB were pulled closer to a reasonable distance and FRET happened prosperously. Notably, thermodynamics in this process was well studied for an in-depth understanding. This work will render the better design of donor–acceptor pairs to overcome the long distances as well as the deep understanding of FRET with spreading applications.