Stochastic Photon Emission from Non-Blinking Upconversion

Article pubs.acs.org/JPCC

Stochastic Photon Emission from Nonblinking Upconversion Nanoparticles Eunsang Lee,† Minhyuk Jung,† Youngeun Han,† Gibok Lee, Kyujin Shin, Hohjai Lee,* and Kang Taek Lee* Department of Chemistry, School of Physics and Chemistry, Gwangju Institute of Science and Technology (GIST), 123 Cheomdan-gwagiro (Oryong-dong), Buk-gu Gwangju, South Korea

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S Supporting Information *

ABSTRACT: Because of their well-known optical properties, upconversion nanoparticles (UCNPs) are regarded as some of the most promising nanomaterials for bioimaging, biosensors, and solar cells. The nonblinking nature of their upconversion emissions has been a particularly beneficial advantage for live-cell imaging. However, the origin of this unique property has never been seriously investigated. We report, for the first time, the observation of stochastic photon emission (SPEM) in core/shell UCNPs (NaYF4:Yb3+,Er3+/NaYF4) on the microsecond and nanosecond time scales, even under continuous irradiation at 980 nm. This SPEM was attributed to slow “upconversion cycles”. We consider that the conventionally reported, nonblinking nature of UCNP emissions can be attributed to the averaging of SPEMs from multiple Er3+ ions and the low temporal resolution of previous observation. The off-time distribution, which possesses kinetics information for the upconversion pathways, was well fitted to a single exponential indicating involvement of a single rate-determining step. The distinct behaviors of the green and red emissions confirm their different photophysical pathways.

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and NaYF4:Yb3+,Er3+) never displayed photoblinking, even at the single-particle level.27,28 The authors only speculated about the origins of this nonblinking behavior, and neither group tried to explain why single UCNPs do not blink. Since these reports, numerous studies were devoted to UCNP imaging, wherein the continuous visualization of UCNP probes is demanded.29 In this article, we report our observation that NaYF4:Yb3+,Er3+/ NaYF4 UCNPs do, in fact, exhibit a very extensive photoblinking on the microsecond and nanosecond time scales. This UCNP blinking originates from extremely slow upconversion cycles during which Er3+ ions do not emit photons. It is noteworthy at this point that the “photoblinking” may not be an accurate term to be used for UCNPs because that word usually represents the intermittent luminescence due to the process that deviates from standard Poisson statistics. Thus, we will use “stochastic photon emission (SPEM)” instead of “photoblinking”, and this change will be clearly justified later in this paper. Finally, the observed green and red emissions, produced via distinct mechanisms, also display different SPEM behaviors. The absence of SPEM in previous studies is attributed to both a substantial number of lanthanide ions (Yb3+ and Er3+) in the UCNPs and the binning effect of signal points.

INTRODUCTION Upconversion nanoparticles (UCNPs), especially the most wellstudied NaYF4:Yb3+,Er3+ nanoparticle system, are known to be extremely photostable. In particular, they never exhibit photoblinking or photobleaching upon continuous excitation with near-IR (980 nm) radiation.1−9 One of their best features, the nonbleaching nature of UCNPs is attributed to the stabilization of the 4f-4f transition of Er3+ through an electronic shielding effect by 5s and 5p electrons.10,11 Therefore, prolonged irradiation on the order of hours does not damage the chemical structure and the optical properties of the UCNPs. This is especially useful for imaging biological samples, whose dynamics requires extended monitoring.12 In contrast, photoblinking is a reversible intermittent emission that occurs even under continuous excitation. For example, such photoblinking phenomenon is common in quantum dots (semiconductor nanocrystals), and it inhibits continuous tracking due to dark periods over a wide range of time scales.13−17 Numerous studies have been attempted to reveal the origin of the photoblinking behaviors of quantum dots,18−25 but a consistent picture or theoretical description has not been established. To our knowledge, only a single study on the photoblinking observed in lanthanide-doped nanoparticles (Eu3+-doped Y2O3) has been published.26 In this case, however, the system was not upconverting with multiphoton absorption, and the blinking time scale was from hundreds of milliseconds to seconds, which is far from the mystery hidden behind “non-blinking upconversion emissions”. In 2009, Park et al. and Wu et al. independently reported the remarkable observation that the lanthanide-doped UCNPs (NaGdF4:Yb3+,Er3+/NaGdF4 © 2017 American Chemical Society

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EXPERIMENTAL METHODS Synthesis of β-NaYF4:Yb3+(20%), Er3+(2%)/β-NaYF4 Core/Shell Nanoparticles. Yttrium-(III) acetate hydrate Received: August 25, 2017 Published: September 1, 2017 21073

DOI: 10.1021/acs.jpcc.7b08509 J. Phys. Chem. C 2017, 121, 21073−21079

Article

The Journal of Physical Chemistry C

directed to the PMT. The green and red emissions were filtered more by band-pass filters. When we acquired only the green emission signals we blocked the red emission acquisition path and v.v. The PMT was connected to a digital oscilloscope (TDS2024C, Tektronix) which was controlled by a computer interface (LabVIEW). The time traces were analyzed by homemade LabVIEW and Matlab codes written especially for the off-time distributions and histograms.

(99.9%), ytterbium-(III) acetate tetrahydrate (99.9%), erbium-(III) acetate hydrate (99.9%), oleic acid (technical grade, 90%), 1-octadecene (technical grade, 90%), ammonium fluoride (≥98%), and sodium hydroxide (≥98%) were purchased from Sigma-Aldrich. All reagents were used as received. The NaYF4:Yb3+,Er3+ UCNPs (cores) were synthesized according to the previously reported procedure.30 Briefly, Y-(CH3COO)3·xH2O (0.78 mmol), Yb-(CH3COO)3·4H2O (0.20 mmol), Er-(CH3COO)3·xH2O (0.02 mmol), oleic acid (6 mL), and 1-octadecene (15 mL) were mixed, heated to 160 °C for 40 min, and cooled to room temperature. Ammonium fluoride (4.0 mmol) and sodium hydroxide (2.5 mmol) in methanol (10 mL) were added, and the methanol was removed by heating. The solution was heated to 300 °C and kept at 300 °C under Ar for 1 h. The UCNPs were precipitated with absolute ethanol (10 mL) and recovered by centrifugation. The nanoparticles were washed with ethanol and dispersed in hexane. In order to coat the cores with β-NaYF4 shell, Y-(CH3COO)3· xH2O (0.5 mmol), oleic acid (6 mL), and 1-octadecene (15 mL) were mixed, heated to 160 °C for 40 min, and cooled to room temperature. After cooling to room temperature, the core UCNPs in hexane (1 mmol) were added. Ammonium fluoride (2.0 mmol) and sodium hydroxide (1.25 mmol) in methanol (5 mL) were added, and the methanol was removed by heating. The solution was heated to 300 °C and kept at 300 °C under Ar for 1 h. The UCNPs were precipitated with absolute ethanol (10 mL) and recovered by centrifugation. Finally, the core/shell UCNPs were washed with ethanol and dispersed in hexane. Measurement of UCNP Emission Time Traces. UCNP films were generated evenly on cover glasses by spin coating. The coverage was controlled by the concentration of the UCNPs in n-hexane. The UCNP imaging setup used in our previous work31 was slightly modified in order to incorporate a photomultiplier tube (PMT, H10492, Hamamatsu) in addition to the electronmultiplying charge-coupled device (EMCCD) camera for the detection of UCNP emission in the visible range (Figure 1). A single-mode CW 980 nm laser was introduced and focused on the UCNP sample by a 60× oil immersion objective lens (NA 1.49, Olympus). UCNPs can be viewed simply as an assembly of many emitters. The emission was collected by the same objective lens, separated into the green (∼525 and ∼545 nm) and red (∼660 nm) emission bands by a dichroic mirror, and selectively

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RESULTS AND DISCUSSION To detect UCNP emissions, the epifluorescence imaging setup was modified as shown in Figure 1. We added a photomultiplier tube (PMT) for photodetection and connected it to a PC-interfaced oscilloscope. Using the PMT instead of the electron-multiplying charge-coupled device (EMCCD) was the breakthrough in this study. The PMT’s fast response to the arrival of photons (a few nanoseconds) is superior to that of EMCCD, which requires a typical exposure of at least a few tens of milliseconds. The UCNP samples were prepared as film on a cover glass. Figure 2 shows the time traces of emissions from UCNPs dispersed on a cover glass under 980 nm CW excitation. In Figure 2a we irradiated UCNP-free regions as a control and observed no significant signals other than the background and occasional small peaks probably due to scattering or dark current. We constructed a histogram for the background intensity and determined the threshold (0.5) above which photon signals were considered genuine. In fact, the noise-like peaks above the threshold in the UCNP-free region were very sparse, and the total peak intensity in Figure 2a was as small as 0.2% that of the genuine UCNP signal peaks with the same laser excitation power. The level of the noisy backgrounds was negligible, and they do not make any contribution to further analysis and the conclusion. The UCNP samples were irradiated with a 980 nm laser, and the upconversion emission signals were collected. As shown in Figure 2b, we observed a number of burst peaks which divide zero-level dark periods (inset and see also Figure S1). The widths of the burst peaks are similar to those of the PMT response profiles, indicating that the bursts themselves are so transient that they could not be further resolved. We assigned each burst peak to the emission of photons from the UCNPs. The number of photons per burst that were emitted can be inferred from the

Figure 1. Schematic diagram of the setup for UCNP SPEM measurements. 21074


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Figure 2. (a) Time trace and intensity histogram of signals from the UCNP-free region (control). Intensity is represented in arbitrary units. (b) Time trace and intensity histogram from UCNP emission. Intensities above 0.5 were treated as emission signals. (Inset) Magnification of the trace between 0 and 20 μs. (c) Time trace and intensity histogram of the UCNP-free region (red) and UCNP emission (black) obtained by binning the 500 data points in 100 ms.

absorb photons and as a result provides more Er3+ ions with chances to emit photons. In Figure 3a and 3b the UCNP emission signals were compared between the two different excitation powers, 0.6 and 0.2 mW. We observed that both the off-times and the intensities of the burst peaks are substantially more affected at the power of 0.6 mW: the higher intensities because of the superposition of single-photon peaks and the shorter intervals between off-times due the increase of emitting Er3+ numbers in the crystal. We also measured the power-dependent SPEM to estimate the number of photons required for the upconversion process using the equation I ∝ Pn, where I is the emission intensity, P the laser excitation power, and n the number of photons absorbed (Figure S2). Remarkably, we obtained n = 1.20 ± 0.03 (simultaneous detection of the red and green), indicating two-photon upconversion. In other words, we confirm that the burst signals indeed originated from UCNPs. The upconversion mechanism of the UCNP can be summarized by incorporating the SPEM behavior observed in this work as follows. Yb3+ ions in the nanoparticle start their upconversion cycles by absorbing 980 nm photons and transfer their energy to adjacent Er3+ ions. The excited Er3+ ions undergo relaxations to the lower states and finally emit visible photons. However, the fraction of such successful emissive cycles might be

intensity histograms in which the lowest peak (one photon) intensity is predominant and other peaks (multiple photons) are relatively rare. This is clearly the first observation of UCNP intermittent emission, which we named as SPEM. Remarkably, when we sum up the raw data (500 points) in 100 ms bins, a continuous emission trace is produced and the overall signals appear to be nonblinking as shown in Figure 2c. At the millisecond or longer time scale, a large number of collected photons are binned, and as a result, the emission is represented as a finite and nearly constant signal far above zero level. This uninterrupted emission time trace represents the nonblinking emissions of UCNPs. Here, we find that the individual Er3+ ions emit a photon only occasionally, but we cannot observe the SPEM due to the binning effect (i.e., low temporal resolution) of our eyes and the cameras. On the other hand, we might also be able to address the contribution of Er3+ concentration in the crystals to the nonblinking nature of UCNPs. However, we noted that the concentration of Er3+ cannot be increased monotonously because of the quenching process known as “cross relaxation” between Er3+ ions at their high concentration.32 On the contrary, the excitation laser powers are directly related to the effective number of Er3+ in UCNPs, namely, the higher power allows more Yb3+ ions to 21075


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Figure 3. (a) Intensity histograms for the green and red emissions at 0.6 mW. (b) Intensity histograms for green and red emissions at 0.2 mW. Insets are the time trace data used for constructing the histograms.

to be no single answer to the detailed mechanism for the red emission, and the pathways vary with excitation power and wavelength.37 In this work, we focus on how the green and red undergo their individual pathways and behave differently as in many other aspects. Thus, we hope we can provide a model that encompasses all pathways. In Figure 4b and 4c, time traces and off-time distributions are shown for the green and red emissions, respectively. Here, the off-time distributions of the two traces were fitted to a single exponential with decay time constants of 1.66 ± 0.03 and 2.59 ± 0.02 μs for the green and red bands, respectively, at an excitation power of 0.6 mW (see Figure S4 for the off-time distribution obtained by the excitation at a lower power, 0.2 mW). This confirms that pathway B takes longer than pathway A because of the nonradiative relaxation between the sequential absorption of photons in B. This is another piece of evidence for mechanistic differences between the green and the red emissions. More importantly, the time required for the upconversion cycle, reflected in the off-time distribution, shows a single-exponential decay for both paths A and B. This means, for each pathway, that there is only one rate-determining step, a transition from a state with the longest lifetime. We hypothesize that the 4I11/2 → 4F7/2 (A) and 4I13/2 → 4F9/2 (B) transitions are rate-determining steps. These hypotheses are justified by (1) the 4I11/2 and 4I13/2, of which the lifetimes are long enough for the second energy transfer from Yb3+ ions to be efficient, and (2) these transitions, which are the distinct part between the two

very low, which is equivalent to stating that the quantum yield is very low. Only after a number of these cycles will the Er3+ emit a photon. During the delay between the consecutive emission events, that Er3+ would be dark, resulting in the dark period of Er3+-SPEM. However, despite the inefficient nature of this upconversion, the behavior of UCNPs appears to be nonblinking because of the low temporal resolution of the detectors (or the binning of data points) and the large number of Er3+ ions in the nanoparticles. The emission in the visible spectral range for NaYF4:Yb3+,Er3+ is divided into three major bands, two green bands (525, 545 nm) and a red band (660 nm) (Figure S3a). We have insisted and supported the idea of different photophysical pathways for the green and red emissions.33,31 In Figure 4a energy diagrams are shown for the two pathways (A and B). In pathway A, two-photon absorption leads simultaneously to the green and red emissions. However, most of the red emission stems from pathway B. The green and red emission pathways were distinguished by various spectroscopic techniques (488 nm one-photon excitation, power dependence, time-resolved emission,33 and stimulated emission depletion31). However, there has been debate on the origin of the red emission: the conventional two-photon mechanism (pathway B in our works) or three-photon mechanism suggested by Berry et al. and Resch-Genger et al.34−36 We also observe multiphoton processes involving even more than four photons at the lowest laser power we can get. Thus, there seems 21076


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Figure 4. Energy diagram for UCNPs and time traces (Figure 3a) for the green (525 and 545 nm) and red (660 nm) emission bands at an excitation power of 0.6 mW. (a) Upconversion pathways of the two major peaks of the UCNPs. Green emission stems from pathway A (left), and most of the red emission originates via pathway B (right). (b) Time trace and off-time histogram for the green emissions. (Inset) Magnification of the trace between 0 s and 20 μs. (c) Time trace and off-time histogram for the red emission. (Inset) Magnification of the trace between 0 s and 20 μs.

function to the time traces of the UCNP emissions (green + red), as in Figure 5. Although the burst peaks are apparently connected to one another in the trace, it turns out that there are no correlations between emitted photons. Instead, the correlation decays very quickly to zero, which indicates that every upconversion emission from the UCNPs is random and the Er3+ ions do not interact with other Er3+ ions, and indeed, are unaware of their presence. Thus far, all of the experimental results and discussion were about film composed of multiple UCNPs. In general, photoblinking or SPEM has any meaning only for single molecules and

mechanisms (A and B), make the substantial difference in the offtimes of the green and red emissions. The autocorrelation analysis of a time trace is useful for characterizing the nature of the signals. The autocorrelation function (A(τ)) is defined as ∞

A (τ ) =

∫−∞ I(t )I(t + τ )dt ∞

∫−∞ I(t )2 dt

(1)

where τ represents the time delay between two data points and I(t) is the intensity of the burst peaks at time t. We applied this 21077



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Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b08509. Time traces and intensity histograms from UCNP emission in different regions, power dependence of UCNP emission intensity; characterization and emission spectrum of β-NaYF4:Yb3+,Er3+ core/shell nanoparticles (UCNPs), off-time histogram at the excitation power of 0.2 mW (PDF)

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AUTHOR INFORMATION

Corresponding Authors

Figure 5. Normalized autocorrelation plotted by using the raw time trace data in Figure 2b. Strong correlation is observed only at τ = 0, which indicates that the emission events (burst peaks) are not correlated but totally stochastic.

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions †

E.L., M.J., and Y.H.: These authors contributed equally to this work.

single particles. We also repeated all measurements with single UCNP particles and obtained essentially the same conclusion (Figure 6). The signal intensity was high enough to be distinguished from the background signals. The off-time distribution was a single-exponential decay with 7.54 ± 0.06 μs. It is noteworthy that for UCNPs composed of numerous dopant ions (Yb3+ and Er3+) it is more reasonable to perceive the emission from single Er3+ rather than from single particles.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS K.T.L. was supported by grants funded by the National Research Foundation (NRF) (NRF-2017R1A2B3005414 and NRF2016R1D1A1B03931644) of S. Korea. H.L. was supported by the Basic Science Research Program (NRF-2016R1D1A1A02937339) of the NRF and a grant from the GIST Research Institute (GRI, 2016).

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CONCLUSIONS In summary, we observed extensive SPEM in the time traces of UCNP emission on the microsecond and nanosecond time scales. The nonblinking nature of the UCNP emissions was attributed to statistically random emissions from multiple Er3+ ions and the insufficient temporal time resolution of previous observation. The off-time distributions, possessing all of the information on the upconversion photophysics, were well fitted to single exponentials, which indicates the presence of single ratedetermining steps that are most likely the transition from the intermediate states (4I11/2 and 4I13/2) before the second energy transfer from Yb3+. The green and red emissions show significantly different off-time distributions even for the SPEM behavior. Note that the SPEM observed in this study does not conflict with the conventionally reported “continuous, nonblinking emission of UCNPs”. On the contrary, the photoblinking of UCNPs might be used for imaging Er3+ ions in the crystals with super-resolution, which requires a stochastic blinking of the probes without photobleaching.38

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Figure 6. (a) Time trace (excitation power 595 mW), (b) intensity histogram, and (c) off-time histogram for the emission of a single UCNP. 21078


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Stochastic Photon Emission from Non-Blinking Upconversion

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