Toward Imaging Defect-Mediated Energy Transfer between Single Nanocrystal Donors and Single Molecule Acceptors

Abstract

Defect-mediated energy transfer is an energy transfer process between midgap electronic states in a semiconductor nanocrystal (NC) and molecular acceptors, such as fluorescent dye molecules. Super-resolution fluorescence microscopy represents an exciting technique for pinpointing the nanoscale positions of lattice defect sites in, for example, a micrometer-sized particle or thin film sample by spatially resolving the location of the acceptor dye molecules with nanometer resolution. Toward this goal, our group performed ensemble-level, time-resolved fluorescence spectroscopy measurements of ZnO NC/Alexafluor 555 (A555) mixtures and calculated that the emissive defect sites are located, on average, 0.5 nm from the NC surface [NilssonZ. N. Nilsson, Z. N. J. Chem. Phys.2021154054704]. However, ensemble-level measurements cannot spatially resolve the defect sites on single particles, nor can they distinguish between surface-adsorbed dye molecules that participate in the energy transfer (EnT) process from those that do not. In this work, we compared the photoluminescence intensity trajectories of 789 isolated, single ZnO NC donors to those of 73 non-specifically bound and five specifically bound ZnO NC/A555 pairs, where the donor and acceptor centroid positions were separated by a distance that was smaller than our localization precision (40 nm). We observed minor fluorescence intensity fluctuations in the donor and defect channels instead of clear anticorrelated intensity fluctuations, which could be explained by (1) the presence of multiple emissive defect sites per NC, (2) donor–acceptor separation distances slightly larger than the Förster radius (R 0 = 3.1 nm; defined as the distance at which EnT is 50% efficient), and/or (3) poor dipole–dipole coupling. The single molecule imaging methodology we developed, an alternating ultraviolet–visible excitation sequence combined with multicolor photon detection, successfully distinguishes specifically bound and non-specifically bound NC/dye pairs and can be applied to study a wide range of hybrid NC/dye energy transfer systems.

Characterizing the Ligand Shell Morphology of PEG-Coated ZnO Nanocrystals Using FRET Spectroscopy

“…The simultaneous donor PL quenching and acceptor PL enhancement in Figure 2d indicate that the photoexcited ZnO NCs induce the A555 fluorescence via a defect-mediated EnT process (Figure 2d). 54,67,74 We observed the same trends in Figure 2 for all of the ZnO NC samples. However, we quantified the EnT efficiency of the NC-dye mixtures using TRPL experiments because the PL lifetime measurements are more accurate than steady-state PL quenching efficiencies; lamp fluctuations can impact the interpretation of steady-state data.…”

“…73 Because the absorption of A555 overlaps with the defect PL (see pink shaded region in Figure 2b), the trapped carriers can also participate in a defect-mediated EnT process with A555 at or near the NC−solution interface, as schematically shown in Figure 2a. 54,67,74 Note, A555 absorbs very little UV light, specifically at photon energies that also excite the ZnO NCs. Hence, the absorption properties of the donor and acceptor allow us to selectively excite the ZnO NC donor in the presence of the acceptor.…”

Getting the Most Out of Fluorogenic Probes: Challenges and Opportunities in Using Single-Molecule Fluorescence to Image Electro- and Photocatalysis

“…Single-molecule fluorescence (SMF) microscopy provides the unique capability to probe individual chemical reactions with millisecond time resolution and nanoscale spatial resolution. − ,− It has been used to measure both differences among the reactivity of catalyst particles prepared within the same batch and to image nanoscale variations in activity across individual particles. However, the technique relies on chemically activated fluorogenic probes that serve as proxies for the reaction of interest.…”