Charge-Driven Fluorescence Blinking in Carbon ... - ACS Publications

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Charge-Driven Fluorescence Blinking in Carbon Nanodots Syamantak Khan, Weixing Li, Narain Karedla, Jan Thiart, Ingo Gregor, Anna M. Chizhik, Joerg Enderlein, Chayan Kanti Nandi, and Alexey I. Chizhik J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b02521 • Publication Date (Web): 10 Nov 2017 Downloaded from http://pubs.acs.org on November 11, 2017

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The Journal of Physical Chemistry Letters

Charge-Driven Fluorescence Blinking in Carbon Nanodots Syamantak Khan1, Weixing Li2, Narain Karedla2, Jan Thiart2, Ingo Gregor2, Anna M. Chizhik2, Jörg Enderlein2,3, Chayan K. Nandi1*, Alexey I. Chizhik2*

AUTHOR ADDRESS 1

School of Basic Sciences, Indian Institute of Technology Mandi, Himachal Pradesh

175001, India 2

University of Göttingen, Third Institute of Physics, 37077 Göttingen, Germany

3

Center for Nanoscale Microscopy and Molecular Physiology of the Brain (CNMPB),

37077 Göttingen, Germany

AUTHOR INFORMATION Corresponding author C.K.N.: Email: [email protected], Phone: +49-551-397723 A.I.C.: Email: [email protected], Phone: +91-1905-237917

ABSTRACT This study focuses on the mechanism of fluorescence blinking of single carbon nanodots, which is one of their key but less understood properties. The results of our

ACS Paragon Plus Environment


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single particle fluorescence study show that the mechanism of carbon nanodots blinking has remarkable similarities with that of semiconductor quantum dots. In particular, the temporal behavior of carbon nanodot blinking follows a power law both at room and at cryogenic temperatures. Our experimental data suggests that static quenching via Dexter type electron transfer between surface groups of a nanoparticle plays a major role in the transition of carbon nanodots to off- or grey states, whereas the transition back to on-states is governed by an electron tunneling from the particle’s core. These findings advance our understanding of the complex mechanism of carbon nanodots emission, which is one of the key steps for their application in fluorescence imaging.

TOC GRAPHICS


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KEYWORDS: carbon nanoparticles, fluorescence blinking, time correlated single photon counting, Dexter type electron transfer, electron tunneling, cryogenic temperature.



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Fluorescence blinking is a general property of most luminescent systems, the study of which allows for unraveling the intriguing photo-physics of various types of quantum emitters1-4. One of the most prominent examples is the complex fluorescence mechanism of semiconductor quantum dots (QDs), which is still not fully understood, even after two decades of intense research of their fluorescence intermittency. In general, the probability densities of both on- and off-time distributions for semiconductor QDs follow a power law over several orders of magnitude in time. The basic physical model that explains such a broad rate distribution of QD blinking attributes the on-off florescence transition to random charging and discharging of the QD core. An excess of charge carriers opens a non-radiative de-excitation channel via Auger relaxation, which leads to either off-states or so-called grey states with an intermediate fluorescence intensity. Typically, fluorescence intensity fluctuations are accompanied by a proportional variation of the excited state lifetime5, 6. Time-correlated single photon counting studies of QDs revealed other possible mechanisms for fluorescence blinking, which involve surface charge-recombination centers. In particular, Klimov and co-workers have shown that QDs may exhibit either excited state lifetime fluctuations with constant emission intensity7 or vice versa8. In the latter case, the authors attribute blinking to charge fluctuations in electron-accepting surface sites, which can intercept hot electrons before they relax into emitting core states. In 2004, Scrivens and co-workers have first demonstrated bright fluorescence from carbon nanoparticles9, which were referred to as carbon nanodots (CNDs) by analogy with semiconductor QDs. Although it has been shown later that their


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fluorescence stems from surface groups10, 11, the emission properties of CNDs turned out to be a unique hybrid combination of those of dye molecules and of semiconductor nanocrystals. In particular, CNDs blinking follows a power law distribution12 similar to that of the blinking of semiconductor QDs2, which suggested that charge trapping or charge redistribution on the surface of the particles trigger their transitions between emissive and dark states. This assumption has been supported by reports on photoswitching of CNDs via electron transfer between different surface groups of the particles,13 and by fluorescence quenching of CNDs using photoinduced electron transfer between CNDs and electron donor/acceptor molecules in solution14. However, the mechanism of spontaneous blinking of CNDs remains unclear. Here, we show that photo-blinking in CNDs is governed by Dexter type electron transfer from fluorescent surface groups to charge accepting surface sites and by electron tunneling from the CND’s core. Our CNDs, which were synthesized according to the method by Ding et al. 15, possess a good solubility in such common solvents as water or methanol and show bright fluorescence in the yellow-red spectral region (figure 1(a)). The ensemble emission spectrum of the particles exhibits a notable independency on excitation wavelength in the whole studied range from 450 to 561 nm (figure 1(a)). Excitation wavelength dependent spectral shift of CNDs fluorescence that has been commonly reported in previous publications16, multichromophoric units11,

18

17

, is caused by excitation of

. Its absence indicates homogeneity of the particles

physico-chemical properties, such as type of surface chemical groups, particle size, shape, etc. The fluorescence quantum yield of CNDs in methanol and water is 83 and 17



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%, respectively. The change of the CNDs’ excited state lifetime both in water and methanol didn’t exceed 10% in the whole range of excitation wavelengths (figure 1(c)). Since fluorescence quantum yield is proportional to excited state lifetime of an emitter19, this result indicates that the quantum yield of CNDs stays nearly constant at different excitation wavelengths. A nearly 20 nm spectral shift of CNDs fluorescence and change of the excited state lifetime by a factor of 5 in water and in methanol (figures 1 (a) and (b)) suggest that particles possess an electric dipole. Nuclear magnetic resonance (NMR, see figure S1 of the Supporting Information) and Fourier Transform Infrared Spectroscopy (FTIR, see figure S2 of the Supporting Information) measurements revealed that CNDs contain C=O, C-OH, CH2 and CH3 surface groups that is consistent with the previous studies10, 15. The NMR data suggests that most of the carbon atoms are sp3 hybridized, which is in contrast to sp2 conjugated carbon, give rise to amorphous structure of the CNDs core20. Figure 1(b) shows a fluorescence spectrum of a single CND dispersed on a clean glass surface. As in the previous studies (see ref.11), the single particle spectra has an asymmetric shape, which indicates that several vibronic transitions are involved in the de-excitation process. Fitting the single CNDs spectra with two Lorentzian functions allowed us to calculate the energy separation between the two vibronic bands. As in the previous studies, the energy band separation lies in the range from 100 - 140 mEv that corresponds to a coupling between electronic transitions and phonons in carbon core of the particles11. However, here, the emission maxima are distributed within a narrow spectral range of near 40 nm, whereas previous single CND spectroscopic studies


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reported on 120 nm distribution width11. Narrowing of the single particle emission spectra distribution width indicates similarity of the local chemical environment around the emissive surface groups21. Single particle polarization measurements showed that all the fluorescent CNDs studied in this work possess a fixed linear excitation transition dipole (see inset in figure 1(b)). This finding implies that each nanoparticle has only one optically active fluorescent surface group, which is directly excited by the laser light without any energy transfer from other surface groups22, 23. Thus, the results of both ensemble and single particle spectroscopic and time resolved characterization measurements show that physico-chemical properties of different single CNDs that were used in this work are nearly identical. This is an essential prerequisite for analysis of their blinking statistics and further possible application in fluorescence microscopy studies. For measuring single CNDs fluorescence time traces at room and cryogenic temperatures, we used our recently developed custom-built wide field microscope with reduced sample drift in a wide temperature range (see ref.24 for details). For maximizing the photon collection efficiency, we used a 1.49 NA oil immersion objective (Apo N, 60×, 1.49 NA, Olympus) for all room temperature studies, and a 0.7 NA air objective (LUCPLFLN, Olympus, 60×, 0.7 NA) for the cryogenic measurements. Fluorescence detection was done using an EMCCD camera (iXon Ultra 897, Andor Technology). Particles were excited with 488 nm continuous laser light. Figure 2(a)-(c) and (d)-(f) show examples of intensity traces which were measured at 300 and 89 K, respectively. As shown by the fluorescence kinetics curves



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(figure S5), the particles exhibited significantly higher photo-stability at cryogenic temperature. Some of the CNDs emitted light for more than 1 hour (see figure 2(d)). The most intriguing result is that, both at room and at cryogenic temperatures, CND fluorescence exhibited multi-step blinking. Recently, multi-state fluorescence at room temperature has been reported for various types of CNDs12, 13, 25. This finding implies that multi-step blinking is unrelated to a specific type of CNDs, but is rather their common feature. Single-step transitions between the off-state and the high-intensity on-state (see figure 2(a) and (e)) suggests that (as has been shown in the recent publication ref.12) the high-intensity on-state corresponds to an emissive state of a single quantum emitter, but not to a superposition of signals from several particles. Subsequent switching of fluorescence to a lower intensity level corresponds to another emissive state of the same CND. We have performed an analysis of on- and off-state durations using the recently developed TrackNTrace software26. Histograms of on- and off-state durations were calculated from time-traces which were measured at room temperature in ambient atmosphere, and in vacuum at low temperature. In all cases, the histograms could be well fitted by a power law (figure 3)

P (t on off ) = At −α ,

(1)

where the probability density P(t) is proportional to the number of blinking events. Equality of the power law exponents α physically means the equality of the rate constants of the on-off switching rates or the average on- or off-states duration.


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Power law distributions suggest that, by analogy with semiconductor quantum dots,4 charge trapping plays a major role in stochastic fluorescence intermittency over a wide temperature range. Moreover, as commonly observed for fluorescence blinking of semiconductor nanocrystals27, a decrease of temperature led to a significant increase of the on-state duration (change of α from -1.9 ± 0.03 to -1.6 ± 0.1), whereas the average duration of dark states exhibited nearly no temperature dependence (1.31 ± 0.03 at room temperature versus 1.2 ± 0.1 at cryo-temperature). This finding also agrees with a recent observation that an electron transfer mechanism is involved in the photoinduced on-off switching of CNDs13. It should be noted that depending on experimental parameters, semiconductor nanocrystals can exhibit more complex blinking statistics that cannot be fitted solely by the power law function and requires for example introduction of an exponential cut-off2,

27-29

. However, such deviation from the pure

power law statistics does not contradict to the general model of the nanocrystal ionization and charge trapping, which is the key point of our comparison between carbon nanodots and semiconductor nanoscrytals. To reveal the blinking behavior of CNDs at the microsecond scale, we performed dual-focus FCS measurement of CNDs in aqueous solution. In figure 4 , autocorrelation functions (red and blue curves ) and a cross-correlation function (purple and green curves) were calculated from data recorded for diffusing CNDs using a dual-focus FCS model, as described in refs. 30, 31. The theoretical fit to the experimental data (figure 4) showed that blinking occurs at time scales as short as few microseconds. Thus, the observed CND blinking distribution extends over seven orders of magnitude in time.



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Correlation of the signal in the millisecond time range allowed us to determine the diffusion constant D of the thermally induced translational diffusion of CNDs in water. From the obtained value of 3.2±0.3*10-6 cm2/s one can deduce the size of CNDs by using the Stokes-Einstein relation rh =

k BT , 6πηD

(2)

where rh is the hydrodynamic radius, kB is the Boltzmann constant, T is the sample temperature, and η is the viscosity of the solution. Equation 2 results in a rh value of 0.9±0.1 nm which is good agreement with the mean size of the particles that was obtained from transmission electron microscopy data (see figure S4 of the Supporting Information). Using a 20 MHz pulsed 488 nm excitation laser and an avalanche photodiode (APD) as photo-detector we measured fluorescence time traces of single CNDs at room temperature with a temporal resolution of 50 ps. This allowed us to measure average excited state lifetimes for different emissive states. The bottom panel in figure 5 shows a fluorescence time trace of a single CND. The photon emission rate of the particle exhibits two different on-states, indicated by horizontal dashed lines. The upper panel of figure shows the histogram of photon detection times plotted separately for the onstates I-V. All the obtained fluorescence decay curves could be well fitted with a monoexponential function (red lines), which along with single-step on-off transitions suggests that each of the curves is related to a single on-state of a single nanoparticle, but not to a superposition of multiple states with rapid switching between them32. The calculated


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values of the excited state lifetime are shown in the top-right corner of every sub-panel. It is remarkable that the lifetime values for the different on-states are the same (within our measurement errors). The inset in the bottom panel of figure 5 shows an average lifetime value of 2 ns for the on-states I-V. All the CNDs which showed multi-level blinking had always a constant excited state lifetime, although the lifetime value is different for different particles (see figures S6 and S7 for more single particle time traces). Variation of fluorescence intensity at constant excited state lifetime is a typical sign that CND emission is modulated by Dexter type electron transfer.33,

34

This is

supported by the notion that chemical groups on the particles’ surface can act as electron traps13. These findings, along with the power law blinking statistics, allow us to suggest a model which describes all the key aspects of fluorescence blinking of CNDs observed up to now. The left panel of figure 6 shows a schematic of the proposed model. The core idea is the presence of multiple surface groups, which act as charge traps with various trapping probabilities. The brightest on-states of a CND correspond to a complete absence of or lowest-rate charge transfer for a given particle. The probability of Dexter electron transfer kET decreases exponentially with the distance r between the donor and acceptor:  − 2r  k ET ∝ J exp ,  L 

(3)



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where L is the sum of the Van der Waals radii between electron donor and acceptor, and J is the spectral overlap integral

33, 34

. Hence, the separation between the

fluorescent surface group and the charge trap is the key factor that determines the probability of an electron transfer. For short distances (near 1 nm or less) between the fluorescent group (D in figure 6) and the charge acceptor (A1), the probability of charge transfer from the donor and back is high. The ratio of the probabilities of non-radiative de-excitation via charge transfer, and radiative recombination at the fluorescent surface group, determine the average fluorescence intensity of a given intensity state. Increased charge transfer results in enhanced quenching of fluorescence and switching of the particle to a so-called grey state, which is also commonly observed for semiconductor nanocrystals32. If an electron acceptor (A2) is located at a longer distance from the fluorescent surface group, to which the charge transfer probability is low, it can lead to rare spontaneous charge trapping for long time periods, which results in a complete off-state of the CND. Averaging of charge transfer activation rates kET (Arrhenius equation), which are exponentially proportional to the activation energy Ea  − Ea  k ET ∝ exp   RT 

(4)

over an exponentially distributed set of rate constants (see equation 3), results in a power law distribution of on-times. However, direct transitions of the charge from the trap back to the donor cannot explain why the average off-time time duration is independent on temperature. In the


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presence of higher energy barriers, which result in stable charge trapping, the most probable mechanism is an electron tunneling from the CND’s core. Due to the delocalized electron cloud in the neutral core of a CND, charge recovery for both donor D and acceptor A2 happen via electron tunneling from and to the core, respectively. When an electron of mass me reaches the barrier, it satisfies the Schrödinger equation 2 −h 2 ∂ Ψ ( x ) =  E e − U ( x )  Ψ ( x ) , 2m e ∂x 2

(5)

where Ψ is the Schrödinger wave function, U(x) is the potential barrier, and E is the total energy of the electron. The Wentzel-Kramers-Brillouin semi-classical approximation allows one to find a solution to equation 5 from which one determines the transition probability rate T(E) through a general 1D barrier as  x2 2me (U ( x ) − Ee )  T (E ) = exp − 2 ∫ dx  . 2 x1 h  

(6)

Because the barrier height U depends on the considered charge trap center, the offtime distribution follows a power law similar to that of the on-time distribution. As a result, the donor charge recovery leads to switching of a particle back to the on-state and to the reactivation of its fluorescence (see “OFF→ON transiRon” in the right panel of figure 6). A similar mechanism of a temperature-independent process has also been observed in semiconductor quantum dots under conditions similar to our experiments, where a charge has been trapped on a surface site27. The charge transfer between the donor and different acceptor centers, that is, transition between on-, off-, and grey states, is modulated by the photo-induced variation of the energy distribution of charge traps. This process has been recently



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observed in SiO2 nanoparticles, where fluorescence also originates from surface centers35, 36. In particular, it has been shown that fluorescence emission can reversibly or irreversibly switch between different surface centers, which led to the modulation of both fluorescence intensity and excited state lifetime. Despite the different role of surface centers in the fluorescence of SiO2 nanoparticles and of CNDs, the photoinduced energy redistribution among different surface groups might hint at a common origin of the emission intensity fluctuations observed in both cases. The fluorescence mechanisms of the on-, off- and grey states are summarized in the right panel of figure 6. In summary, based on our time-resolved single particle studies, we proposed a model of fluorescence blinking in CNDs. The model relies on Dexter type electron transfer from a fluorescent center of the CND to charge acceptors on the particle’s surface. The stochastic dynamics of this charge transfer is governed by photo-induced energy fluctuations of the charge traps, which lead to a modulation of the charge transfer probability. Thus, CNDs possess a unique and complex combination of photophysical properties mixing characteristics of semiconductor nanocrystals, dye molecules, and dielectric nanoparticles. Understanding the mechanism of CNDs’ blinking is one of the key requirements for their successful application in fluorescence imaging.

ASSOCIATED CONTENT Supporting Information. Experimental methods and supporting figures (PDF)


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AUTHOR INFORMATION *Corresponding author C.K.N.: Email: [email protected], Phone: +49-551-397723 A.I.C.: Email: [email protected], Phone: +91-1905-237917 NOTES The authors declare no competing financial interest.

ACKNOWLEDGEMENTS Funding by the German Science Foundation (DFG, SFB 937, project A14) and Department of Biotechnology (Project No: BT/PR4067/BRB/10/1128/2012), India is gratefully acknowledged. Advanced Materials research Centre (AMRC) of IIT Mandi HP India is also gratefully acknowledged. J.E and A.M.C. are grateful to HFSP for financial support. Part of this work (J.E.) was supported by the Cluster of Excellence and DFG Research “Center Nanoscale Microscopy and Molecular Physiology of the Brain.”



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FIGURES

Figure 1. (a) Fluorescence spectra of CNDs in water (solid black curve), methanol (color solid curves), and on the glass surface in air (dashed curve). The spectra in methanol were measured at different excitation wavelengths to verify independence of the emission spectrum on excitation wavelength. (b) Black solid curve: fluorescence spectrum of a single CND dispersed on the surface of a glass cover slide. Blue and red curves are Lorentzian fits to individual vibronic bands and a total fit, respectively. Vertical dotted lines show the position of the emission maxima of the other single particle fluorescence spectra. Inset shows consecutive images of the same sample area recorded by using an azimuthally polarized laser beam (scanning direction


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up−down). Each double-lobe pattern corresponds to the same CND. The images show fluorescence intermittency and single-step photobleaching (image (3)) of the particle. Image “model” shows a theoretically calculated excitation pattern for identical experimental conditions assuming a fixed linear horizontal dipole. The orientation of the dipole is indicated by the double arrow. (c) Excited state lifetime of CNDs in water (blue solid circles, right axis) and methanol (red solid circles, left axis) versus excitation wavelength. Red- and blue-shaded areas show the errors of the measurements.



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Figure 2. Fluorescence time traces of single CNDs at 300 K ((a)-(c)) and 89 K ((d)-(f)). The gradual fluctuation of fluorescence intensity in figure (f) is caused by a slight drift of the sample.


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Figure 3. Measured distributions of the off- and on-state durations of single CNDs (blue circles): (a,b) at 300 K and (c,d) at 89 K. The red line is a power law fit to the experimental data. The statistics was collected from (a) 4350, (b) 4783, (c) 974, and (d) 1041 CNDs. The data is shown in the same scale in figure S8 of the Supporting Information.



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Figure 4. The two autocorrelation functions (red and blue curves) and a crosscorrelation function (purple and green curves) were calculated from data recorded for laterally diffusing CNDs using two focus FCS setup. The correlation functions were fitted with the two focus FCS model, as described in refs. 20 and 21.


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Figure 5. Bottom panel: fluorescence time trace of a single CND (black curve). The grey shaded areas indicate subsequent emission bursts (I-V). The dashed horizontal lines indicate the fluorescence intensities of two distinct on-states. Upper panel: fluorescence decay curves, which correspond to the emission bursts I-V. Inset in bottom panel: fluorescence lifetime obtained by fitting the decay curves of the different emission bursts. The dashed line shows the average lifetime value of 2.0 ns for all the five bursts. More single CND fluorescence time traces are shown in figures S6 and S7 of the Supporting Information.



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Figure 6. Model describing the blinking behavior of CNDs. Left panel: Schematic of the charge transfer corresponding to different emission bursts in a fluorescence time trace. Right panel: Collection of fluorescence mechanisms corresponding to the on-, off- and grey states.


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