Luminescence Dynamics of Silica-Encapsulated Quantum Dots During Optical Trapping

Abstract

The trade-off between photobrightening and photobleaching controls the emission stability of colloidal quantum dots. This balance is critical in optical trapping configurations, where irradiances that confine and simultaneously excite the nanocrystals in the focal region cannot be indefinitely lowered. In this work, we studied the photobrightening and bleaching behaviors of two types of silica-encapsulated quantum dots excited upon two-photon absorption in an optical trap. The first type consists of alloyed CdSeZnS quantum dots covered with a silica shell. We found that the dynamics of these as-prepared architectures are similar to those previously reported for bare surface-deposited quantum dots, where thousands of times smaller irradiances were used. We then analyzed the same quantum dot systems treated with an extra intermediate sulfur passivating shell for the better understanding of the surface traps influence in the temporal evolution of their emission in the optical trap. We found that these latter systems exhibit better homogeneity in their photodynamic behavior compared to the untreated ones. These features strengthen the value of quantum dot preparations in optical manipulation as well as for applications where both long and maximal emission stability in physiological and other polar media are required.

Photoluminescence Activation of Organic Dyes via Optically Trapped Quantum Dots

“…During optical trapping experiments, the lasers provide an average power of 90 mW in the spot corresponding to a flux of 2.8 × 10 25 photons/cm 2 ·s (see Methods for spot dimensions). Trapped QDs show PL through TPE as previously reported. ,,,,, Under the conditions reported here, Rh6G and TRITC dyes are not optically excited because our photon flux is deficient for TPE of organic dyes, generally requiring 10 27 –10 29 photons/cm 2 ·s provided by pulsed sources at similar wavelengths. , …”

“…Trapped QDs show PL through TPE as previously reported. 10,16,19,20,48,49 Under the conditions reported here, Rh6G and TRITC dyes are not optically excited because our photon flux is deficient for TPE of organic dyes, generally requiring 10 27 −10 29 photons/cm 2 •s provided by pulsed sources at similar wavelengths. 50,51 As shown in Figure 3a, a trapped QD@SiO 2 system can be moved in the micrometer range within the chamber walls by means of a micrometer drive, hence making it possible to evaluate the emitted PL throughout several thicknesses, z, of optically active medium.…”

“…To perform optical trapping experiments, QD@SiO 2 was dispersed in ethanol or water, where the nanoparticles form small aggregates or clusters, as characterized elsewhere . The measured hydrodynamic diameter of the QD@SiO 2 aggregates was 0.95 ± 0.49 μm, and the number of nanoparticles contained in each cluster was estimated to be ∼10 2 .…”

“…Again, in the presence of 2 μM Rh6G, the intensity does not depend on z for the QD 585 @SiO 2 system because of the small spectral overlap (orange dots, Figure d). The I (λ) decrease when the QD 527 @SiO 2 cluster separates from the coverslip (green dots) may be due to photodynamic bleaching …”

“…The I(λ) decrease when the QD 527 @SiO 2 cluster separates from the coverslip (green dots) may be due to photodynamic bleaching. 48 Deconvoluted signals for QD 527 @SiO 2 in a 2 μM Rh6G ethanol solution show that the intensity from the QD 527 @SiO 2 component strongly attenuates also in the far-field (hollow blue dots). On the other hand, the re-emitted signal from the Rh6G component (hollow red dots) increases with the distance, which is expected due to the increasing number of dye molecules that fill the gap from the QD cluster to the coverslip as z grows.…”