Quantum dot single molecule study
(Nanowerk Highlights) Research group Prof. Yinghu and Prof. Preston Snow from the UIC chemistry department have reported single molecule studies of energy transfer using a quantum dot donor with an organic dye acceptor.
Quantum semiconductor dot-dye conjugates as shown in Figure 1(A) are increasingly being used in alternative energy applications to increase the efficiency of photovoltaic devices. Moreover, they can also be applied to biological sensing and imaging.
This combination is very strong due to the smooth absorption of light by the quantum dots (QD), which, in turn, transfer energy to the dye via the well-known “FRET” mechanism. A plus is the fact that the optical properties of the QD can be tuned by synthesizing smaller or larger dots. This size dependence is caused by the phenomenon of quantum confinement, which is a manifestation of the Heisenberg Uncertainty Principle.
The report focuses on the phenomenon of intermittent fluorescence, or “flickering”, which makes dyes and dots emit like the twinkling starry sky at night under a single molecule microscope. The QDs and dyes flicker due to different photophysical mechanisms, and since this applies to energy transfer, the researchers found that both chromophores must be in the “active” state to observe dye emission from the FRET. This lowers the conversion of absorbed photons to dye emission, which affects some applications.
For example, dye photobleaching is strongly 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 is not so good for energy harvesting, as the throughput for converting solar energy into other forms is reduced by as much as 95% as shown by the statistical analysis shown in Figure 1(C).
Fortunately for the alternative energy industry, the group demonstrated that suppression of donor QD flicker minimizes energy loss, which is achieved by a simple surface treatment method.
The group also presented new revelations about the statistical property of quantum dot flickering, which to date has been largely described as following a power law distribution as shown in Figure 2(A). This means that the probability of observing a QD “on” is proportional to the reciprocal of how long it takes, which is a very unusual observation in science.
A new statistical analysis procedure developed by the group, in which the size of the histogram bins is proportional to the exponential distribution of the data, Figure 2(B), allows for greater time resolution. As a result, the CdSe/CdZnS QDs appear to be partially lognormally flickering which may be similar to a bell curve as shown in Figure 2(C).
The results help unravel the mechanism of QD flickering, which the researchers propose may be due to the distribution of charge carrier trapping states, each of which has a different barrier to capture electrons or holes.
The author’s report appears in Journal of Physical Chemistry Letters (“Fluorescence Intermittency Quantum Dot–Organic Dye Conjugates: Implications for Alternative Energy and Biological Imaging”), and supported by the University of Illinois Chicago and the American Chemical Society Petroleum Research Fund. The first author, Hashini Chandrasiri, was assisted by fellow UIC graduate students Haoran Jing and Thilini Perera.
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