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Photophysical behaviors of single fluorophores localized on zinc oxide nanostructures.

Fu Y, Zhang J, Lakowicz JR - Int J Mol Sci (2012)

Bottom Line: In this report we studied photophysical behaviors of single fluorophores in proximity to zinc oxide nanostructures by single-molecule fluorescence spectroscopy and time-correlated single-photon counting (TCSPC).Single fluorophores on ZnO surfaces showed enhanced fluorescence brightness to various extents compared with those on glass; the single-molecule time trajectories also illustrated pronounced fluctuations of emission intensities, with time periods distributed from milliseconds to seconds.The fluorescence fluctuation dynamics were found to be inhomogeneous from molecule to molecule and from time to time, showing significant static and dynamic disorders in the interfacial electron transfer reaction processes.

View Article: PubMed Central - PubMed

Affiliation: Center for Fluorescence Spectroscopy, Department of Biochemistry and Molecular Biology, School of Medicine, University of Maryland School of Medicine, 725 Lombard Street, Baltimore, MD 21201, USA; E-Mails: jianzhang@umaryland.edu (J.Z.); jlakowicz@umaryland.edu (J.R.L.).

ABSTRACT
Single-molecule fluorescence spectroscopy has now been widely used to investigate complex dynamic processes which would normally be obscured in an ensemble-averaged measurement. In this report we studied photophysical behaviors of single fluorophores in proximity to zinc oxide nanostructures by single-molecule fluorescence spectroscopy and time-correlated single-photon counting (TCSPC). Single fluorophores on ZnO surfaces showed enhanced fluorescence brightness to various extents compared with those on glass; the single-molecule time trajectories also illustrated pronounced fluctuations of emission intensities, with time periods distributed from milliseconds to seconds. We attribute fluorescence fluctuations to the interfacial electron transfer (ET) events. The fluorescence fluctuation dynamics were found to be inhomogeneous from molecule to molecule and from time to time, showing significant static and dynamic disorders in the interfacial electron transfer reaction processes.

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Histograms and exponential fits of the off time durations (green open circle: molecules on glass; red open square: molecules on ZnO surfaces). The solid line over open circles show a single exponential fit for single Cy5 molecules dispersed on glass: τoff = 1.73 ± 0.02 ms. The inserts show log−log scale off time distributions.
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f4-ijms-13-12100: Histograms and exponential fits of the off time durations (green open circle: molecules on glass; red open square: molecules on ZnO surfaces). The solid line over open circles show a single exponential fit for single Cy5 molecules dispersed on glass: τoff = 1.73 ± 0.02 ms. The inserts show log−log scale off time distributions.

Mentions: The duration of the “off” states extends over a broad range from subseconds to seconds. To analyze the single molecule interfacial electron transfer dynamics that is associated with the fluctuation of the single-molecule fluorescence trajectories, we have performed statistical analysis on the stochastic durations of the dark states in which the high ET activity takes place. A method similar to the previously used to analyze triplet blinking was used: a threshold was determined at a level of 2-fold standard deviation with respect to the mean background during the off time histogram analysis to distinguish between the states as described elsewhere [38,39]. A combined off-time distribution from more than 20 molecules is used herein. The time duration of the dark states are then obtained. The histogram of the off time on bare glass is fitted with a single-exponential curve and yields an average off time (τoff) of 1.73 ± 0.02 ms. The log-log distribution of Cy5 molecules on bare glass substrate can be fitted to a straight line (the insert in Figure 4). We can describe this off time distribution using a power law function p(τoff) = p0τoff–α0ff, with α0ff being the power law exponent. The single molecule power exponent for the off-time statistics is sensitive to the molecule environment and/or the molecule structure. It is obvious from the linear log-log plot that the observed off-times on glass coverslips covers a range of times and can be described using a power law distribution. The α0ff value of 1.50 of Cy5 molecules in the absence of ZnO is in agreement with other organic fluorophores obtained on glass substrates [40], which suggests that the molecular system is well behaved and the blinking is due to a single process. It is interesting that for single molecules on ZnO surfaces, multi-exponential behavior is observed in the dark time distribution shown in Figure 4. This multi-exponential distribution suggests that the interfacial ET involves complex processes that cannot be defined by a static rate constant and the rate changes from time to time, and lead to a dramatic change in “off” times. The electron transfer reactivity can be reflected by the fluorescence emission fluctuation. As electron transfer reactivity fluctuates, fluorescence shows dramatic fluctuations and gives striking bright and dark states. The intermittency and fluctuation of the single-molecule fluorescence are then attributed to the differences of the reactivity of interfacial electron transfer at the single molecule/ZnO nanoparticle interfaces [35,41,42]. The “off” periods in the fluorescence trajectories are accounted for by an electron transfer process with high activity quenching the fluorescence and the “on” periods are owing to low activity in ET process.


Photophysical behaviors of single fluorophores localized on zinc oxide nanostructures.

Fu Y, Zhang J, Lakowicz JR - Int J Mol Sci (2012)

Histograms and exponential fits of the off time durations (green open circle: molecules on glass; red open square: molecules on ZnO surfaces). The solid line over open circles show a single exponential fit for single Cy5 molecules dispersed on glass: τoff = 1.73 ± 0.02 ms. The inserts show log−log scale off time distributions.
© Copyright Policy - open-access
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC3472795&req=5

f4-ijms-13-12100: Histograms and exponential fits of the off time durations (green open circle: molecules on glass; red open square: molecules on ZnO surfaces). The solid line over open circles show a single exponential fit for single Cy5 molecules dispersed on glass: τoff = 1.73 ± 0.02 ms. The inserts show log−log scale off time distributions.
Mentions: The duration of the “off” states extends over a broad range from subseconds to seconds. To analyze the single molecule interfacial electron transfer dynamics that is associated with the fluctuation of the single-molecule fluorescence trajectories, we have performed statistical analysis on the stochastic durations of the dark states in which the high ET activity takes place. A method similar to the previously used to analyze triplet blinking was used: a threshold was determined at a level of 2-fold standard deviation with respect to the mean background during the off time histogram analysis to distinguish between the states as described elsewhere [38,39]. A combined off-time distribution from more than 20 molecules is used herein. The time duration of the dark states are then obtained. The histogram of the off time on bare glass is fitted with a single-exponential curve and yields an average off time (τoff) of 1.73 ± 0.02 ms. The log-log distribution of Cy5 molecules on bare glass substrate can be fitted to a straight line (the insert in Figure 4). We can describe this off time distribution using a power law function p(τoff) = p0τoff–α0ff, with α0ff being the power law exponent. The single molecule power exponent for the off-time statistics is sensitive to the molecule environment and/or the molecule structure. It is obvious from the linear log-log plot that the observed off-times on glass coverslips covers a range of times and can be described using a power law distribution. The α0ff value of 1.50 of Cy5 molecules in the absence of ZnO is in agreement with other organic fluorophores obtained on glass substrates [40], which suggests that the molecular system is well behaved and the blinking is due to a single process. It is interesting that for single molecules on ZnO surfaces, multi-exponential behavior is observed in the dark time distribution shown in Figure 4. This multi-exponential distribution suggests that the interfacial ET involves complex processes that cannot be defined by a static rate constant and the rate changes from time to time, and lead to a dramatic change in “off” times. The electron transfer reactivity can be reflected by the fluorescence emission fluctuation. As electron transfer reactivity fluctuates, fluorescence shows dramatic fluctuations and gives striking bright and dark states. The intermittency and fluctuation of the single-molecule fluorescence are then attributed to the differences of the reactivity of interfacial electron transfer at the single molecule/ZnO nanoparticle interfaces [35,41,42]. The “off” periods in the fluorescence trajectories are accounted for by an electron transfer process with high activity quenching the fluorescence and the “on” periods are owing to low activity in ET process.

Bottom Line: In this report we studied photophysical behaviors of single fluorophores in proximity to zinc oxide nanostructures by single-molecule fluorescence spectroscopy and time-correlated single-photon counting (TCSPC).Single fluorophores on ZnO surfaces showed enhanced fluorescence brightness to various extents compared with those on glass; the single-molecule time trajectories also illustrated pronounced fluctuations of emission intensities, with time periods distributed from milliseconds to seconds.The fluorescence fluctuation dynamics were found to be inhomogeneous from molecule to molecule and from time to time, showing significant static and dynamic disorders in the interfacial electron transfer reaction processes.

View Article: PubMed Central - PubMed

Affiliation: Center for Fluorescence Spectroscopy, Department of Biochemistry and Molecular Biology, School of Medicine, University of Maryland School of Medicine, 725 Lombard Street, Baltimore, MD 21201, USA; E-Mails: jianzhang@umaryland.edu (J.Z.); jlakowicz@umaryland.edu (J.R.L.).

ABSTRACT
Single-molecule fluorescence spectroscopy has now been widely used to investigate complex dynamic processes which would normally be obscured in an ensemble-averaged measurement. In this report we studied photophysical behaviors of single fluorophores in proximity to zinc oxide nanostructures by single-molecule fluorescence spectroscopy and time-correlated single-photon counting (TCSPC). Single fluorophores on ZnO surfaces showed enhanced fluorescence brightness to various extents compared with those on glass; the single-molecule time trajectories also illustrated pronounced fluctuations of emission intensities, with time periods distributed from milliseconds to seconds. We attribute fluorescence fluctuations to the interfacial electron transfer (ET) events. The fluorescence fluctuation dynamics were found to be inhomogeneous from molecule to molecule and from time to time, showing significant static and dynamic disorders in the interfacial electron transfer reaction processes.

Show MeSH
Related in: MedlinePlus