Single molecule studies of quantum dot - dye energy transfer

(Nanowerk Spotlight) The research groups of Prof. Ying Hu and Prof. Preston Snee of the UIC chemistry department have reported single molecule studies of energy transfer using a quantum dot donor with an organic dye acceptor.
Semiconductor quantum dot-dye conjugates such as shown in Figure 1(A) are increasingly used in alternative energy applications to enhance the efficiency of photovoltaic devices. Furthermore, they can also be applied for biological sensing and imaging.
The combination is powerful due to the facile absorption of light by the quantum dots (QDs) that, in turn, transfers energy to the dye via the well-known “FRET” mechanism. Adding value is the fact that the optical properties of QDs can be tuned by synthesizing smaller or larger dots. This size dependence is due to the quantum confinement phenomenon, itself a manifestation of the Heisenberg Uncertainty Principle.
Quantum dot-dye conjugates
Figure 1. (A) Quantum dot-dye conjugates. (B) Blinking of quantum dots, Cy5 dye, and the dye when conjugated to the QD. (C) Statistical analyses of the dye blinking via FRET from the QD reveals perturbed behavior that may positively or negatively affect various applications. (Image courtesy of the researchers)
The report centers on the phenomenon of fluorescence intermittency, or “blinking”, that makes emissive dyes and dots appear like a twinkling starry sky at night under a single molecule microscope. QDs and dyes blink due to different photophysical mechanisms, and as it applies to energy transfer, the researchers found that both chromophores must both be in the “on” state to observe dye emission from FRET. This lowers the conversion of absorbed photons into dye emission, which affects several applications.
For example, dye photobleaching is substantially suppressed, which is clearly visible in the lower panel of Figure 1 (B). This is an advantage for single molecule biological sensing and imaging. However, the news isn’t so good for energy harvesting, as the throughput for converting solar energy into other forms is reduced by as much as 95% as demonstrated by the statistical analyses shown in Figure 1(C).
Fortunately for the alternative energy industry, the group demonstrated that suppression of the blinking of the QD donor minimizes the loss of energy, which is achieved by a simple surface treatment method.
Histogram of quantum dot blinking appears to conform to a power law distribution
Figure 2. (A) Histogram of quantum dot blinking appears to conform to a power law distribution. (B) The exponential histogramming method. (C) An exponential histogram of the same dataset from (A) reveals lognormal behavior. (Image courtesy of the researchers)
The group also presented new revelations on the statistical nature of quantum dot blinking, which heretofore has largely been described as following a power law distribution as shown in Figure 2(A). This means that the probability of observing the QD being in the “on” state is proportional to the inverse of how long it’s on, which is a very uncommon observation in science.
A new statistical analyses procedure developed by the group, whereby the bin sizes of a histogram are in proportion to the exponentially distributed of data, Figure 2(B), enabled a greater time resolution. As a result, CdSe/CdZnS QDs were shown to blink in part with a lognormal probably which is akin to a bell curve as shown in Figure 2(C).
The results help unravel the mechanism of QD blinking, which the researchers propose may be due to the presence of a distribution of charge carrier trap states, each of which has a different barrier for capturing electrons or holes.
The authors’ report appears in the Journal of Physical Chemistry Letters ("Fluorescence Intermittency of Quantum Dot–Organic Dye Conjugates: Implications for Alternative Energy and Biological Imaging"), and was supported by the University of Illinois Chicago and the American Chemical Society’s Petroleum Research Fund. The first author, Hashini Chandrasiri, was assisted by fellow UIC graduate students Haoran Jing and Thilini Perera.
Provided by the University of Illinois Chicago

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