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Förster Resonance Energy Transfer between Quantum Dot Donors and Quantum Dot Acceptors.

Chou KF, Dennis AM - Sensors (Basel) (2015)

Bottom Line: The unique optical properties of QDs provide certain advantages and also specific challenges with regards to sensor design, compared to other FRET systems.The fundamentals of FRET within a nominally homogeneous QD population as well as energy transfer between two distinct colors of QDs are discussed.Examples of successful sensors are highlighted, as is cascading FRET, which can be used for solar harvesting.

View Article: PubMed Central - PubMed

Affiliation: Department of Biomedical Engineering, Boston University, Boston, MA 02215, USA. kfchou@bu.edu.

ABSTRACT
Förster (or fluorescence) resonance energy transfer amongst semiconductor quantum dots (QDs) is reviewed, with particular interest in biosensing applications. The unique optical properties of QDs provide certain advantages and also specific challenges with regards to sensor design, compared to other FRET systems. The brightness and photostability of QDs make them attractive for highly sensitive sensing and long-term, repetitive imaging applications, respectively, but the overlapping donor and acceptor excitation signals that arise when QDs serve as both the donor and acceptor lead to high background signals from direct excitation of the acceptor. The fundamentals of FRET within a nominally homogeneous QD population as well as energy transfer between two distinct colors of QDs are discussed. Examples of successful sensors are highlighted, as is cascading FRET, which can be used for solar harvesting.

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Impact of excess donor on spectral results and time-resolved PL. (a) Steady-state PL spectra of mixed green and red, donor and acceptor CdTe QDs with three different donor: acceptor ratios. Spectra shown are for QDs mixed in the absence of Ca2+ and QDs mixed in the presence of Ca2+, which caused the QDs to associate with each other in close enough proximity to induce FRET; (b,c) Time-resolved PL decay curved of donor green (b) and acceptor red (c) QDs in pure green or red samples and in mixed samples in the presence of Ca2+. Reprinted with permission from [76]. Copyright (2008) American Chemical Society.
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sensors-15-13288-f016: Impact of excess donor on spectral results and time-resolved PL. (a) Steady-state PL spectra of mixed green and red, donor and acceptor CdTe QDs with three different donor: acceptor ratios. Spectra shown are for QDs mixed in the absence of Ca2+ and QDs mixed in the presence of Ca2+, which caused the QDs to associate with each other in close enough proximity to induce FRET; (b,c) Time-resolved PL decay curved of donor green (b) and acceptor red (c) QDs in pure green or red samples and in mixed samples in the presence of Ca2+. Reprinted with permission from [76]. Copyright (2008) American Chemical Society.

Mentions: In addition to the various physical donor-acceptor orientations that are possible because of the multivalency of both the donor and acceptor in QD-QD devices, we must also consider the photophysical implications of this structure. In QD-FP or QD-organic dye FRET devices, the most successful sensor designs display multiple acceptors per QD donor. These devices yield increasing FRET efficiencies with each additional acceptor, plateauing once ~4–6 acceptor fluorophores have been successfully attached to the donor QD [22,54]. In contrast, the published QD-QD FRET systems described here often utilize an excess of donor QDs, presumably to increase the amount of the excitation light absorbed by the donor QDs. One can see in an example in Figure 16a that in the absence of FRET (no Ca2+, dashed lines) the extra donor QDs increase the donor emission peak intensity with nominal impact on the acceptor peak intensity. In the presence of Ca2+, however, the red and green QDs aggregate, bringing them in close enough proximity for FRET. In that case, the excess donor QDs do indeed facilitate increased enhancement of the acceptor emission. When the donor time-resolved PL is examined, one can see that the donor lifetime is shortest with the least excess of donor QDs (Figure 16b). This is to say that the excess of donor QDs actually decreases the FRET efficiency, consistent with Equation (2) and Figure 3. The decrease in efficiency is offset by the improvement in the overall PL signal as more incident light is absorbed by donor dots, resulting in higher total acceptor PL. One can also see in Figure 16c that the acceptor lifetime lengthens with the increase in the number of donor QDs per acceptor [76]. It seems that although less of the total energy absorbed by the donors is transferred to acceptors (Figure 16b), more of the acceptor excitation originates through energy transfer rather than through direct acceptor excitation (Figure 16c). In other words, although the FRET efficiency goes down with the increase in the number of donors per acceptor, the total amount of energy transferred appears to increase. The increase in donor concentration also likely also promotes an increase in donor-to-donor homotransfer, perhaps before or in lieu of energy transfer to the intended acceptor. The QD-QD FRET field is ripe for systematic investigations into the interplay between the donor and acceptor absorption cross-sections, the donor-acceptor ratio, the FRET efficiency, and the ratio between the acceptor and donor emission intensities in response to an analyte. While the studies to date have shown that there is merit to the concept of QD-QD FRET-based sensing, thorough investigation is needed to facilitate concerted optimization of QD-QD FRET sensor design, including optimization of the donor:acceptor ratio.


Förster Resonance Energy Transfer between Quantum Dot Donors and Quantum Dot Acceptors.

Chou KF, Dennis AM - Sensors (Basel) (2015)

Impact of excess donor on spectral results and time-resolved PL. (a) Steady-state PL spectra of mixed green and red, donor and acceptor CdTe QDs with three different donor: acceptor ratios. Spectra shown are for QDs mixed in the absence of Ca2+ and QDs mixed in the presence of Ca2+, which caused the QDs to associate with each other in close enough proximity to induce FRET; (b,c) Time-resolved PL decay curved of donor green (b) and acceptor red (c) QDs in pure green or red samples and in mixed samples in the presence of Ca2+. Reprinted with permission from [76]. Copyright (2008) American Chemical Society.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4507609&req=5

sensors-15-13288-f016: Impact of excess donor on spectral results and time-resolved PL. (a) Steady-state PL spectra of mixed green and red, donor and acceptor CdTe QDs with three different donor: acceptor ratios. Spectra shown are for QDs mixed in the absence of Ca2+ and QDs mixed in the presence of Ca2+, which caused the QDs to associate with each other in close enough proximity to induce FRET; (b,c) Time-resolved PL decay curved of donor green (b) and acceptor red (c) QDs in pure green or red samples and in mixed samples in the presence of Ca2+. Reprinted with permission from [76]. Copyright (2008) American Chemical Society.
Mentions: In addition to the various physical donor-acceptor orientations that are possible because of the multivalency of both the donor and acceptor in QD-QD devices, we must also consider the photophysical implications of this structure. In QD-FP or QD-organic dye FRET devices, the most successful sensor designs display multiple acceptors per QD donor. These devices yield increasing FRET efficiencies with each additional acceptor, plateauing once ~4–6 acceptor fluorophores have been successfully attached to the donor QD [22,54]. In contrast, the published QD-QD FRET systems described here often utilize an excess of donor QDs, presumably to increase the amount of the excitation light absorbed by the donor QDs. One can see in an example in Figure 16a that in the absence of FRET (no Ca2+, dashed lines) the extra donor QDs increase the donor emission peak intensity with nominal impact on the acceptor peak intensity. In the presence of Ca2+, however, the red and green QDs aggregate, bringing them in close enough proximity for FRET. In that case, the excess donor QDs do indeed facilitate increased enhancement of the acceptor emission. When the donor time-resolved PL is examined, one can see that the donor lifetime is shortest with the least excess of donor QDs (Figure 16b). This is to say that the excess of donor QDs actually decreases the FRET efficiency, consistent with Equation (2) and Figure 3. The decrease in efficiency is offset by the improvement in the overall PL signal as more incident light is absorbed by donor dots, resulting in higher total acceptor PL. One can also see in Figure 16c that the acceptor lifetime lengthens with the increase in the number of donor QDs per acceptor [76]. It seems that although less of the total energy absorbed by the donors is transferred to acceptors (Figure 16b), more of the acceptor excitation originates through energy transfer rather than through direct acceptor excitation (Figure 16c). In other words, although the FRET efficiency goes down with the increase in the number of donors per acceptor, the total amount of energy transferred appears to increase. The increase in donor concentration also likely also promotes an increase in donor-to-donor homotransfer, perhaps before or in lieu of energy transfer to the intended acceptor. The QD-QD FRET field is ripe for systematic investigations into the interplay between the donor and acceptor absorption cross-sections, the donor-acceptor ratio, the FRET efficiency, and the ratio between the acceptor and donor emission intensities in response to an analyte. While the studies to date have shown that there is merit to the concept of QD-QD FRET-based sensing, thorough investigation is needed to facilitate concerted optimization of QD-QD FRET sensor design, including optimization of the donor:acceptor ratio.

Bottom Line: The unique optical properties of QDs provide certain advantages and also specific challenges with regards to sensor design, compared to other FRET systems.The fundamentals of FRET within a nominally homogeneous QD population as well as energy transfer between two distinct colors of QDs are discussed.Examples of successful sensors are highlighted, as is cascading FRET, which can be used for solar harvesting.

View Article: PubMed Central - PubMed

Affiliation: Department of Biomedical Engineering, Boston University, Boston, MA 02215, USA. kfchou@bu.edu.

ABSTRACT
Förster (or fluorescence) resonance energy transfer amongst semiconductor quantum dots (QDs) is reviewed, with particular interest in biosensing applications. The unique optical properties of QDs provide certain advantages and also specific challenges with regards to sensor design, compared to other FRET systems. The brightness and photostability of QDs make them attractive for highly sensitive sensing and long-term, repetitive imaging applications, respectively, but the overlapping donor and acceptor excitation signals that arise when QDs serve as both the donor and acceptor lead to high background signals from direct excitation of the acceptor. The fundamentals of FRET within a nominally homogeneous QD population as well as energy transfer between two distinct colors of QDs are discussed. Examples of successful sensors are highlighted, as is cascading FRET, which can be used for solar harvesting.

Show MeSH
Related in: MedlinePlus