Fluorescence quantum efficiency of CdSe/ZnS quantum dots embedded in biofluids: pH dependence

Abstract

The radiative quantum efficiency (η) of CdSe/ZnS core-shell quantum dots (QDs) embedded in synthetic oral fluid was measured using a thermal lens (TL) technique. TL transient measurements were performed using the mode-mismatched dual-beam (excitation and probe) configuration. Thermal optical characterization of CdSe/ZnS QDs was performed for two different core sizes (3.9 and 5.1 nm) incorporated into synthetic saliva with different potential of hydrogen (pH 4–8) values. The thermal diffusivity (D) and average emission wavelength (⟨λem⟩) results are approximately independent of the pH of the solutions evaluated. The fractions of absorbed energy converted into heat (φ) and η are dependent on both the fluid pH and core size of the CdSe/ZnS core-shell QDs. The dependence on pH was also evidenced by fluorescence measurements, which corroborate the results obtained by the thermal lens technique.

Optical characterization of core–shell quantum dots embedded in synthetic saliva: Temporal dynamics

“…The fluorescence intensity increases as the medium in which the QDs were embedded changes from an acidic to a neutral (or alkaline) fluid. The same fluorescence behavior was obtained for human saliva [17], and the dependence of  on pH values has been reported [17,18,44]. The average  values obtained by the TL method are presented in Table 2 for both nonfunctionalized and functionalized QDs.…”

“…The quantum efficiency of QDs is dependent on the pH value of the medium in which QDs are immersed [1,17,18]. The cause of the full decrease in the fluorescence intensity or quantum efficiency of QDs is not understood but possibly can be attributed to defects created on the surface of QDs that contribute to nonradiative recombination sites at the surface [15,16].…”

“…The average  values obtained by the TL method are presented in Table 2 for both nonfunctionalized and functionalized QDs. For the CdSe/ZnS QDs studied using the TL technique (or fluorescence spectra results), the highest  obtained was for the most alkaline pH medium (pH ~8.3) [17]. Acids and bases solutions by Arrhenius definition [45] present hydrogen (H + ) and hydroxide (OH -) ions, respectively.…”

“…In contrast, cancer patients present lower salivary pH levels [30]. The use of QDs as a pH-dependent sensor is a further possibility for applications of QDs embedded in biofluids because the pH of human saliva may vary in different disease states [17].…”

“…The shell provides a physical barrier between the core and the surrounding environment, making the core less sensitive to environmental changes in the nanocrystals due to photooxidation [14] and improving the radiative quantum efficiency () by passivating the nonradiative recombination sites at the surface [15,16]. The  parameter can depend on the potential hydrogen (pH) value of the medium in which the QDs are immersed [1,17,18]. pH is capable of affecting the structure and activity of both molecules and nanoparticles.…”