Single-Particle Fluorescence Intensity Fluctuations of Carbon Nanodots

Jan 8, 2014 - between multiple fluorescence intensity levels indicative of ... blinking to the background level suggesting single-molecule behavior...
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Letter pubs.acs.org/NanoLett

Single-Particle Fluorescence Intensity Fluctuations of Carbon Nanodots Somes K. Das, Yiyang Liu, Sinhea Yeom, Doo Young Kim,* and Christopher I. Richards* †

Department of Chemistry, University of Kentucky, 505 Rose Street, Lexington, Kentucky, United States S Supporting Information *

ABSTRACT: Fluorescent carbon nanodots (CNDs) were synthesized in oxidized and reduced forms and were analyzed at the single-particle level. Images of single CNDs at different excitation energies revealed significant heterogeneity in the lower energy trap sites between particles. We observed that a high percentage of reduced CND particles transitioned between multiple fluorescence intensity levels indicative of multichromophoric systems. Despite this behavior, individual CNDs exhibit single-step photobleaching and transient blinking to the background level suggesting single-molecule behavior. KEYWORDS: Carbon nanodot, single particle fluorescence, nanostructure, fluorescence jumps and quenched states

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subsequent chemical or electrochemical treatments lead to changes in photophysical properties such as quantum yields.28 Several reports show that chemical functional groups formed at the surface of CNDs are responsible for the observed fluorescence.5,29,30 One consistent feature shown in these studies is that the reduction of CNDs results in improved quantum yields and in changes to the absorption and emission spectra.31,32 In contrast to most organic fluorophores33 and quantum dots,34 previous reports indicate that CNDs do not exhibit any fluorescence intermittency.5 However, ensemble emission spectroscopic studies of CNDs often reveal that the fluorescence spectrum shifts depending on excitation wavelength. This suggests that fluorescent CNDs might include mutichromophoric units.35,34,39 The presence of multiple chromophores could result from either different conjugation length of the CND core or different chemical groups on the CND surface.36,39 A primary question regarding CND fluorescence is whether the multiple emissive species are contained within a single carbon particle or result from separate particles. Additionally, details regarding the photostability and fluorescence intermittency of CNDs have been masked in ensemble fluorescence studies due to inhomogeneity in size, shape, and chemical defects. To address these questions we characterized the fluorescence properties of oxidized and reduced CNDs at the single particle level. Steady-state fluorescence measurements were conducted for CNDs dispersed in aqueous solutions. Both oxidized and reduced CNDs reveal a red shift in the emission as the

arbon-based nanomaterials such as carbon nanotubes, graphene, and fullerenes have received significant attention because of their remarkable properties and potential use in a variety of applications. Fluorescent carbon nanodots (CNDs) are emerging nanomaterials that show great promise in bioimaging,1−6 optical sensing,7−10 photocatalysis,11−14 and photovoltaic applications.15−18 These nanometer-sized carbon dots exhibit extremely bright fluorescence and photostability. One aspect of fluorescent CNDs that has generated considerable excitement is their cost-effective synthesis and excellent biocompatibility, which cannot be achieved with conventional inorganic quantum dots. Several studies have demonstrated the use of CNDs in bioimaging applications by taking advantage of their high emission rates to visualize particles in Caenorhabditis elegans (C. elegans),19 mammalian cells,3,20−22 and Drosophila.23 Despite the obvious benefits of such a bright and nontoxic fluorescent agent that can be readily incorporated into biological systems, the fluorescence mechanism of CNDs has not been explored in detail. Furthermore, studies to determine the role of structure, chemical defects, and surface passivation on the emissive properties of CNDs show a dependence on the method used to synthesize CNDs. These methods include laser ablation of graphite,5 chemical oxidation of candle soots20 and carbon fibers,24 synthesis in aqueous solutions,25 and patterning from graphene.26 As a result, wide variations exist in the literature regarding the photoluminescence properties of CNDs. However, several consistent features have emerged. For example, the fluorescent properties of CNDs are sensitive to solvent pH and surface passivation.27 It was also reported that chemical oxidation or reduction of CNDs strongly influences the emissive properties, at least partially, through changes in the surface functional groups of CNDs. Thus, synthetic methods, surface passivation, and © 2014 American Chemical Society

Received: October 13, 2013 Revised: December 18, 2013 Published: January 8, 2014 620

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Figure 1. A comparison of single particle emission from reduced and oxidized CNDs isolated in a polymer film. (A) Fluorescence image of oxidized CNDs with widefield excitation (561 nm). (B) The same field of view with 640 nm excitation. (C) Merged image of both 561 and 640 nm excitation showing the fluorescence originates from separate particles. (D) Fluorescence image of 561 nm excitation of reduced particles. (E) The same field of view of the reduced particles in (D) with 640 nm excitation. (F) Merged image of both 561 and 640 nm excitation of the reduced particles showing the different color fluorescence originates from different particles.

excitation wavelength shifts to lower energy (Supporting Information Figure 1). The observed wavelength dependent emission is consistent with several previous reports for similar CND systems.31,35,36 However, at higher energy excitation we observed a fluorescence maximum at ∼525 nm that did not shift over an excitation range of 300−450 nm. At lower energy excitation (>480 nm), the emission spectra shifted to the red as the excitation wavelength increased. Compared to the oxidized CNDs, the fluorescence spectra of the reduced CNDs were significantly blue shifted. UV−vis absorbance spectra of unoxidized CNDs showed a continuous absorption from 200 to 800 nm indicating no defined band gap. Once oxidized, CNDs show a relatively sharp absorption peak at 230 nm (Supporting Information Figure 2). The shift from continuous to sharp absorption is due to the breakage of sp2-bonded carbon conjugation leading to an increased band gap. Oxidized CNDs also showed substantial absorbance at longer wavelengths indicating the presence of lower energy states either from larger CND cores or oxygenated defect-related emissive traps on the particle surface.37 In order to determine the origin of the fluorescence observed at multiple wavelengths in ensemble measurements, we performed fluorescence measurements on isolated single particles of oxidized and reduced CNDs. Separate samples of oxidized and reduced CNDs were diluted and mixed with poly(vinyl alcohol) (PVA) and were spun-cast on a transparent glass substrate. Wide-field fluorescence images were obtained with excitation at 488, 561, and 640 nm. Figure 1A,B shows representative fluorescence images of oxidized CND samples excited at 561 and 640 nm, respectively. Similar numbers of well-dispersed individual particles were observed in both cases. When Figure 1A,B were overlaid (Figure 1C), we saw no overlap indicating that none of these carbon particles absorbs

and emits at both wavelengths. This observation implies that the red shifted emission of the ensemble spectra at emission wavelengths of >550 nm for oxidized CNDs (Supporting Information Figure 1A) are due to the heterogeneity of carbon particles, that is, carbon particles containing chromophores at different energy levels. A fluorescence image taken with 488 nm excitation showed fewer visible particles (∼60% of that of 561 excitation) (Supporting Information Figure 3). The ensemble emission spectra of oxidized CNDs showed a maximum fluorescence intensity at 488 nm excitation. Combined with the fact that fewer numbers of single fluorescent particles were visible, this suggests that some particles do not exhibit sufficient brightness with 488 nm excitation to be detected at the single particle level. Additionally, excitation of the same field of view at 488 and 561 nm showed no overlap between particles when the separate images were overlaid (Supporting Information Figure 3), indicating that excitation with 488, 561, and 640 nm leads to emission that occurs from completely different particles. To determine if the green-emitting fluorescent particles (525 ± 25 nm) excited at 488 nm could also emit in the red (609 ± 27 nm), we detected the emission in both regions during 488 nm excitation. When we overlaid the images of green and red emission for the same field of view, we observed about 10% overlap (Supporting Information Figure 4A−C). This indicates that a small fraction of the particles likely transfer excitation energy to lower-energy sites, which are possibly surface states of the particle. Similar results were observed for the 561 excitation and emissions at 609 ± 27 and 697 ± 37 nm wavelength ranges (Supporting Information Figure 4D−F). Figure 1D−F show wide-field fluorescence images of electrochemically reduced CNDs at 561 and 640 nm excitation. In contrast to the oxidized CNDs, the relative number of 621

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Figure 2. Single particle intensity versus time trajectories extracted from wide-field imaging (100 ms exposure) with 561 nm excitation (78 W/cm2) of CNDs. (A) Representative oxidized particle showing one emission intensity level and single step bleaching to the background level in the upper time trace (85%) and multilevel fluorescence in the lower time trace (15%). (B) Representative reduced particle showing one emission intensity level and single step bleaching to the background level in the upper time trace (25%) and single particle emission exhibiting multiple fluorescence intensity levels before permanently photobleaching in the lower trace (75%).

Figure 3. Single molecule intensity versus time trajectories (50 ms bins) with 561 nm excitation (0.3 kW/cm2) of carbon nanodots. (A) Representative oxidized particle showing one emission intensity level and single step bleaching to the background level. (B) Single particle emission exhibiting multiple fluorescence intensity levels before permanently bleaching. Seventy percent of the oxidized molecules exhibited the behavior shown in (A) and the remaining exhibited the behavior in (B). (C) Representative reduced particle showing one emission intensity level and single step bleaching to the background level. (D) Single particle emission exhibiting multiple fluorescence intensity levels before permanently bleaching. In contrast to the oxidized particles, only 40% of the reduced particles exhibited single level fluorescence (C) and 60% exhibited multilevel emission (D).

different sizes of carbon nanoparticles or different emitting trap sites.29 We also recorded fluorescence intensity versus time traces of individual oxidized and reduced CND particles during wide field illumination with a 561 nm laser excitation (78 W/cm2) and EMCCD detection (Figure 2). Oxidized (Figure 2A) and reduced (Figure 2B) CNDs showed distinct behavior. Of the 56 oxidized CNDs studied, 85% exhibited typical single molecule behavior with a constant emission rate (emission intensity per unit time) and single step bleaching (Figure 2A). Approximately 15% of the oxidized particles exhibited fluorescence intensity fluctuations between multiple intensity

particles visible with 640 nm excitation (Figure 1E) is greatly reduced as compared to 561 nm excitation (Figure 1D). These observations are consistent with the blue shift seen in the ensemble fluorescence spectra of the reduced CNDs (Supporting Information Figure 1). Oxidized CNDs have a core−shell structure where the core of conjugated sp2-bonded carbon nanodomains is surrounded by oxygenated functional groups (e.g., carboxylic and carbonyl groups). TEM images show as-grown CND particles have a size distribution of 2−10 nm (Supporting Information Figure 5). Previous reports proposed that the multiple color emission is likely due to 622

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Figure 4. Photostability measurements of CNDs with 561 nm excitation under continuous higher intensity (0.3 kW/cm2) confocal excitation (561 nm). Histogram of recorded bleaching time of individual molecules for (A) oxidized and (B) reduced particles show 3.6 and 7.5 s respectively as obtained by single exponential fittings.

excitation intensity, the percentage of particles exhibiting multiple fluorescence intensity levels was very different between oxidized and reduced samples. At 561 nm excitation, only 30% of the 49 oxidized CND particles studied showed multiple fluorescence levels compared to 60% of the 51 reduced particles studied. Fluorescence intensity often jumps up to higher and down to lower levels, and molecules had as few as one fluorescence intensity level and as many as five discrete intensity levels separate from the background level. Multilevel fluorescence from conjugated polymer systems has been attributed to defects that lead to fluorescence quenching.38−40 Many individual CND particles show a similar behavior with fluorescence intensities jumping between multiple levels. The fluorescence at each level lasts for various time scales and includes intervals of time when the intensity drops to the background level. This is often characterized as fluorescence blinking. These observations are in contrast to an earlier report5 using scanning confocal microscopy that showed no blinking for CNDs. As our experiments monitor individual molecules continuously and record arrival times of all detected photons, we are able to resolve fluorescence intermittency that would not typically be observed using scanning confocal microscopy. Another interesting observation is that the fluorescence intensity for molecules showing single level fluorescence is typically lower than that of multistep fluctuations, as can be seen in Figure 3. The lower emission rate indicates a lower fluorescence quantum yield. Combined with our observation that a higher percentage of reduced CND particles exhibited multistep fluorescence fluctuations (60% for reduced CNDs versus 30% for oxidized CNDs), this implies that oxygenated defects could act as fluorescent quenchers. This interpretation is consistent with the reduced quantum yields observed for oxidized CNDs.31 We also measured the relative photostability of oxidized and reduced CNDs with low (78 W/cm2) and high (0.3 kW/cm2) excitation intensities (561 nm) (Figure 4). Histograms of the average survival times during high intensity excitation were fit to single exponential decay curves showing photobleaching times of 3.6 and 7.5 s for oxidized and reduced CNDs, respectively (Figure 4A,B). For lower intensity excitation, the survival times before photobleaching were 28 s for the reduced CNDs and 14 s for the oxidized CNDs (Supporting Information Figure 7). The survival times of both oxidized and reduced CNDs became shorter at higher excitation intensities, indicative of more frequent occurrence of photoinduced processes. The extended survival time of reduced

levels (Figure 2A). The reduced CNDs showed a different behavior where of the 51 particles studied 75% exhibited multilevel fluorescence levels while only 25% showed single level sustained fluorescence (Figure 2B). Similar percentages were obtained when we restricted our analysis to the first 10 s of the time traces indicating that differences in photostability between reduced and oxidized CNDs did not impact our measurement of multiple fluorescence levels. The presence of multiple levels of fluorescence was defined by the analysis of intensity distribution histograms and the presence of sustained (multiple frame) duration at a particular fluorescence intensity and not transient single-frame fluctuations (Supporting Information Figure 6). The origin for the difference in the emission behaviors between oxidized and reduced CNDs is not clear but might involve the removal of the emissive traps caused by reduction. In all cases individual particles showed single-step photobleaching. This indicates that the emission comes from a single chromophore or highly coupled multichromophores similar to previously reported conjugated polymers or dendritic systems.38−40 Typically, higher fluorescence intensities were observed for reduced particles compared to oxidized particles, which is consistent with previous observations that the quantum yield of CNDs increased when they were chemically reduced.31,32 Movies of individual particles revealed clear differences between oxidized and reduced particles. The reduced particles appeared to exhibit substantially more fluorescence intermittency (Supporting Information Movie 1). Closer examination of individual time traces revealed that the fluctuations in fluorescence intensity for the reduced particles were in fact transitions between different levels of fluorescence intensity and not necessarily a transition to a nonemissive dark state (Figure 2). The discrete intensity jumps observed at low excitation intensity (78 W/cm2) indicate the presence of multiple fluorescence intensity levels from a single particle which have previously been observed in conjugated polymer multichromophoric systems38−40 and quantum dots.41 There have also been reports of carbon-based systems such as graphene exhibiting multichromophoric behavior.42 To further investigate the fluorescence behavior of single CND particles, additional experiments were conducted with higher intensity excitation (0.3 kW/cm2). APDs were used to detect fluorescence signals with higher temporal resolution. Similar to low intensity excitation, single particles of both oxidized and reduced CND particles showed a mix of single or multilevel fluorescence intensities (Figure 3). At this higher 623

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CND particles showed multiple levels, while the oxidized particles predominantly showed a single level. A possible explanation for this is that after the initial excitation the energy is transferred from the higher energy absorbing site to a lower energy emissive site in the oxidized CND particles. In contrast, when CNDs are reduced the low-energy emissive traps were entirely or partially removed, blocking energy-transfer pathways. Consequently, the emission for reduced CNDs is likely to be from the originally excited chromophores. The presence of the emissive traps or quenching states would likely be dependent on the synthetic methods and the treatment of CNDs.

CNDs implies that the transition to a photobleaching pathway is less probable when oxygenated chemical groups are removed from CND surfaces via the reduction process We also examined the particles at much higher excitation intensities (1 kW/cm2) and observed similar behavior of multilevel versus single level fluorescence from isolated particles (Figure 5). We were able to achieve emission rates of more

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ASSOCIATED CONTENT

S Supporting Information *

Detailed methods and supplemental figures. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 5. Single particle intensity versus time trajectory under high intensity green laser excitation (1 kW/cm2). (A) Ten millisecond bin of the time trace showing detected emission intensity as high as 150 000 counts/sec and exhibiting at least four distinct levels of fluorescence. (B) One millisecond bin of the region from 4.2 to 4.6 s of the time trace shown in (A). Even at the highest intensity levels the molecule transiently returns to the background level indicating single fluorophore behavior as evidenced by the blinking events near 4.4 s in (B).

AUTHOR INFORMATION

Corresponding Authors

*(D.Y.K.) E-mail: [email protected]. Phone: 859-2575597. *(C.I.R.) E-mail: [email protected]. Phone: 859-2180971. Author Contributions

S.K.D., D.Y.K., and C.I.R. designed experiments and wrote the manuscript. S.K.D., Y.L., and S.Y. performed experiments. All authors have given approval to the final version of the manuscript.

than 150 000 counts per second for the highest intensity level of single CND particles exhibiting multiple intensity level fluorescence (Figure 5A). The lowest levels of fluorescence often ranged from 20 000 to 25 000 counts per second. As can be seen in the representative time trace the fluorescence intensity jumps between more than five discrete intensity levels. Zooming into a narrow time window (4.2−4.5 s) with a faster time scale (1 ms) binning shows that even at the highest level of fluorescence the particle can undergo a transition to a dark state in a single step. This is clearly depicted in Figure 5B near the 4.4 s mark where the fluorescence intensity drops from 150 000 counts per second to the background level and then subsequently returns back to previous level. While time traces of these particles exhibit single step photobleaching indicative of a single molecule, we also performed photon antibunching measurements that verified that single particles behaved as single photon emitters (Supporting Information Figure 8). Although the origin of the fluorescence is not yet entirely understood in CNDs, there is mounting evidence that emission arises from both an intrinsic bandgap resulting from confined sp2 conjugation in the core of CNDs and extrinsic fluorescence resulting from lower energy traps that can be either directly excited or excited by energy transfer from intrinsic band.19 Because the band gap depends on the size, shape, and fraction of the sp2 domains, tunable fluorescence emission can be achieved by either controlling the domain size of sp 2 conjugation or modifying the chemical groups formed on the surface of carbon dots.24 Additionally, our observation of brighter single particle emission from reduced samples is also consistent with previous reports of higher quantum yields for reduced carbon dots. We observed multiple fluorescence intensity levels of individual CNDs. These observations suggest the possibility that single carbon dots can possess multiple chromophoric units associated with the CND core and oxygenated defectrelated emissive traps. Interestingly, the majority of the reduced

Notes

The authors declare no competing financial interest.

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