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.

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With spreading applications of fluorescent quantum dots (QDs) in biomedical fields in recent years, there is increasing concern over their toxicity. Among various factors, surface ligands play critical roles. Previous studies usually employed QDs with different kinds of surface ligands, but general principles were difficult to be obtained since it was hard to compare these surface ligands with varied chemical structures without common features. Herein, the physicochemical properties of two types of CdTe QDs were kept very similar, but different in the surface ligands with mercaptoacetic acid (TGA) and 3-mercaptopropionic acid (MPA), respectively. These two types of homologous ligands only had a difference in one methylene group (-CH-). The interactions of the two types of CdTe QDs with bovine serum albumin (BSA), which was one of the main components of cell culture, were studied by fluorescence, UV-vis absorption, and circular dichroism spectroscopy. It was found that the fluorescence quenching of BSA by CdTe QDs followed a static quenching mechanism, and there was no obvious difference in the Stern-Volmer quenching constants and binding constants. The thermodynamic parameters of the two types of QDs were similar. BSA underwent conformational changes upon association with these QDs. By comparing the cytotoxicity of these two types of QDs, TGA-capped QDs were found to be less cytotoxic than MPA-capped QDs. Besides, in the presence of serum proteins, the cytotoxicity of the QDs was reduced. QDs in the absence of serum proteins had a higher internalization efficiency, compared with those in the medium with serum. To the best of our knowledge, this is a rare study focusing on surface ligands with such small variations at the biomolecular and cellular levels. These findings can provide new insights for the design and applications of QDs in complex biological media.

The interaction between benzophenone (BP) and bovine serum albumin (BSA) was investigated by the methods of fluorescence spectroscopy combined with UV-Vis absorption and circular dichroism (CD) measurements under simulative physiological conditions. The experiment results showed that the fluorescence quenching of BSA by BP was resulted from the formation of a BP-BSA complex and the corresponding association constants (Ka) between BP and BSA at four different temperatures had been determined using the modified Stern-Volmer equation. The enthalpy change (ΔH) and entropy change (ΔS) were calculated to be -43.73 kJ mol(-1) and -53.05 J mol(-1) K(-1), respectively, which suggested that hydrogen bond and van der Waals force played major roles in stabilizing the BP-BSA complex. Site marker competitive experiments indicated that the binding of BP to BSA primarily took place in site I (sub-domain IIA). The conformational investigation showed that the presence of BP decreased the α-helical content of BSA and induced the slight unfolding of the polypeptides of protein, which confirmed some micro-environmental and conformational changes of BSA molecules.

CdTe QDs functionalized with different groups showed different effects on mitochondrial membrane fluidity.