Lanthanide-Doped Nanoparticles for Stimulated Emission Depletion

Article Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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Lanthanide-Doped Nanoparticles for Stimulated Emission Depletion Nanoscopy Stefan Krause,*,† Mikkel Baldtzer Liisberg,† Satu Lahtinen,‡ Tero Soukka,‡ and Tom Vosch*,† †

Nano-Science Center/Department of Chemistry, University of Copenhagen, Universitetsparken 5, 2100 Copenhagen, Denmark Department of Biotechnology, University of Turku, Kiinamyllynkatu 10, 20520 Turku, Finland

‡

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S Supporting Information *

ABSTRACT: Lanthanide-based photoluminescent emitters have gained strong attention over the past years because they provide certain advantages over organic fluorophores. Especially their excellent photostability has given rise to efforts in applying lanthanides for stimulated emission depletion (STED) nanoscopy. While STED has been demonstrated for dual lanthanide-based upconversion nanoparticles and single lanthanide ions embedded in yttrium aluminum garnet, a proof of the general lanthanide applicability for STED nanoscopy is still missing. Here we show that doping sodium yttrium fluoride (NaYF4) nanocrystals with either dysprosium or europium ions leads to working Stokes-side STED labels. The vast number of states inherent to most of the lanthanide ions should also allow the discovery of other lanthanides as potential Stokes-side STED labels in similar or alternative hosts. KEYWORDS: stimulated emission, nanoscopy, lanthanides, nanoparticles, photostability

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sodium yttrium fluoride, NaYF4), are well-suited for STED nanoscopy.18−20 In this contribution, we demonstrate a simpler, more general approach using direct excitation of lanthanide ions embedded in NaYF4 nanoparticles for STED nanoscopy. NaYF4 has been widely used as host lattice for photon upconverting lanthanide nanocrystals because of the good chemical stability and relatively low phonon energy that help to minimize multiphonon relaxation processes.21−23 Direct excitation of the lanthanide ions, combined in this case with Stokes-side depletion, will enable a large range of possible excitation, emission, and depletion wavelengths. We demonstrate our approach by using dysprosium and europium. Conceptually, our approach can be extended to other lanthanide ion. For this, one should look at the electronic state diagrams of each specific lanthanide and try out a combination of excitation, emission and depletion wavelengths.17,24−27 A good way to start this exploration is by checking the depletion efficiency of a specific lanthanide ion transition in D2O (see Figure S1). Although it has been shown before that STED on a single lanthanide ion is achievable,16 doping with a larger number of lanthanide ions in a surface functionalized nanoparticle28 will drastically reduce the required excitation intensity. Alternatively, our findings also suggest that our concept can pave the way for the creation of single lanthanide ion complexes with strongly absorbing antenna ligands as STED labels, given that the antenna ligands possess the required photostability.29

INTRODUCTION Optical nanoscopy has allowed researchers to probe structures and dynamics of the building blocks of life below the diffraction limit.1−5 The key to reach subdiffraction resolution is understanding and controlling the intrinsic photophysics of the labels. Depending on the type of optical nanoscopy technique used, this could mean stochastically switching a subpopulation of emitters on and off5 or sharpening the spontaneous emission point spread function by stimulated emission depletion (STED).6 For the latter case, a second laser beam with a doughnut shape is superimposed on the diffraction-limited excitation beam which shrinks the effective spontaneous emission point spread function due to stimulated emission in the high intensity areas. The depletion wavelength should be chosen carefully to avoid other excited-state absorption processes.7,8 Besides the right depletion wavelength, photobleaching or photoconversion of the labels should be minimal, because this will limit the number of images obtainable from the sample.9−12 While most labels are subject to photobleaching, there is a limited number of labels that are extremely photostable because of embedment into an inert host material. Examples of such labels are nitrogen or silicon vacancies in diamond13,14 or lanthanide ions embedded in yttrium aluminum garnet (YAG) crystals.15,16 While nitrogen vacancies require enormous excitation and depletion intensities and have a specific, broad absorption and emission, lanthanidebased emitters constitute a whole class of emitters with a variety of narrow transitions over the ultraviolet, visible, and near-infrared range.17 Recent studies demonstrated that upconversion nanoparticles (UCNPs), consisting of a sensitizer ion (e.g., ytterbium) and a lanthanide ion capable of upconversion (e.g., thulium) embedded in a host matrix (e.g., © XXXX American Chemical Society

Received: July 5, 2019 Accepted: August 8, 2019

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DOI: 10.1021/acsanm.9b01272 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials Scheme 1. Schematic Illustration of the Synthesis Procedure of the NaYF4:Dy3+ or Eu3+ (3%) Nanoparticlesa

a

Further details can be found in Materials and Methods and in ref 30.

Figure 1. (A) TEM image of NaYF4:Dy3+ (3%) nanoparticles. (B) NaYF4:Dy3+ (3%) nanoparticle diameter distribution (n = 100 particles). The reported values are the average of the length and width for each particle. (C) Relevant part of the electronic state diagram of Dy3+ indicating the excitation (blue arrows), detected emission (orange arrow), and depletion wavelengths (dark red arrow). (D) Excitation (blue, λem > 515 nm) and emission (red, λex = 454 nm) spectrum of NaYF4:Dy3+ (3%) nanoparticles. The sample was prepared by letting a droplet of nanoparticle solution dry out in order to create a dense particle film.

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part of the energy-state diagram of Dy3+,17 while Figure 1D gives the excitation and emission spectrum. Considering the energy-state diagram and excitation spectrum, we decided to perform direct excitation of Dy3+ combining three output wavelengths of a continuum laser (449, 452, and 473 nm). For Dy3+, these excitation wavelengths will transfer the Dy3+ ions from the 6H15/2 ground state to the 4I15/2 and 4F9/2 excited states. The 4I15/2 state will depopulate nonradiatively to the 4 F9/2 state. From the 4F9/2 state, emission will occur at 572 nm

RESULTS AND DISCUSSION

NaYF4:Dy3+ Nanoparticles. The synthesis of NaYF4:Dy3+ (3%) nanoparticles resulted in slightly elongated spheres with an average diameter of 25.4 ± 1.6 nm. Scheme 1 illustrates the synthesis process; further details can be found in Materials and Methods and in the article by Palo et al.30 An exemplary transmission electron microscopy (TEM) image and a histogram of the diameter distribution can be found in panels A and B of Figure 1, respectively. Figure 1C shows the relevant B


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ACS Applied Nano Materials

Figure 2. (A) Intensity time trace of the emission of a single NaYF4:Dy3+ (3%) nanoparticle, without and with 748 nm coillumination. The excitation intensity was 2 MW/cm2 (449, 452, and 473 nm); the depletion intensity was 320 MW/cm2 (748 nm), and the bin time was 50 ms. The high and low intensity levels were used to calculate the intensity ratios in panel B. (B) Intensity ratios as a function of the depletion laser intensity. The line represents a 1/(1 + I/Is) fit to the intensity ratio data. (C) Confocal image of NaYF4:Dy3+ (3%) nanoparticles spin coated from solution on a glass coverslip. (D) STED image of the same region as in panel C. The nanoparticles were excited using 449, 452, and 473 nm (2 MW/cm2 total intensity) and depleted at 748 nm (320 MW/cm2). The pixel dwell time is 1 ms for panels C and D. (E and F) Intensity cross section profiles of the dashed lines 1 and 2 in panels C and D, respectively. Confocal data is represented in blue while STED data is plotted in red. The numbers give the fwhm value for a Gaussian fit (confocal) or a Lorentzian fit (STED), respectively. (G) Confocal image of a region of close-lying NaYF4:Dy3+ (3%) nanoparticles spin coated from solution on a glass coverslip using a different excitation laser. (H) STED image of the same region as Figure 2G. The particles in panels H and G were excited using the 454.6 nm line from a CW argon ion laser (12.6 MW/cm2 total intensity) and depleted at 748 nm (320 MW/cm2). The pixel dwell time is 10 ms for panels G and H. The scale bar is 1 μm.

(4F9/2 to 6H13/2), 660 nm (4F9/2 to 6H11/2) and 748 nm (4F9/2 to 6H9/2). The only laser available to us with sufficient intensity for depletion was a Ti:sapphire laser (690−1000 nm), so we decided to use the 748 nm transition (4F9/2 to 6H9/2) to deplete the 4F9/2 state of Dy3+. To check the depletion efficiency, the two diffraction-limited spots of the excitation (449, 452, and 473 nm) and depletion (748 nm) beams were overlapped on a single nanoparticle, and the decrease in emission upon increasing the depletion intensity (I) was measured for a single nanoparticle. Figure 2A shows an example of a single NaYF4:Dy3+ (3%) nanoparticle intensity trace with alternating blocking and unblocking of the depletion laser. (A similar experiment was also performed for Dy3+ ions in D2O, which can be found in Figure S1.) Figure 2B shows the relative luminescence intensity, which follows a 1/(1 + I/ Is) dependence as a function of the depletion intensity. This is in agreement with the reported depletion behavior of organic dyes in CW STED experiments.31 Although, we are using

pulsed excitation and depletion, the high repetition rate of the excitation and depletion laser (∼80 MHz) in comparison to the long luminescence lifetime of the NaYF4:Dy3+ (3%) nanoparticles (∼0.75 ms, see Figure S2) allows for approximating the experimental conditions as being a CW experiment. From fitting the data in Figure 2B with the abovementioned dependency, we extract a saturation intensity value IS (value at which the initial luminescence intensity drops to 50%) of 7.1 MW/cm2 for NaYF4:Dy3+ (3%). This IS value is similar to what has been reported for organic fluorophores (e.g., 11 MW/cm2 for crimson beads).32 However, our IS value is still significantly higher than those reported for UCNPs.20 Our IS value could potentially be improved by optimizing the lanthanide concentration in the nanoparticles and by investigating the role of concentration-mediated crossrelaxation.18,20 At high depletion intensities (e.g., 640 MW/ cm2, see Figure S3) there is a small probability of generating upconversion emission from the 748 nm depletion beam. This C


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Figure 3. (A) TEM image of NaYF4:Eu3+ (3%) nanoparticles. (B) NaYF4:Eu3+ (3%) nanoparticle diameter distribution (n = 107 particles). The reported values are the average of the length and width for each particle. (C) Relevant part of the electronic state diagram of Eu3+ indicating the excitation (blue arrow), detected emission (orange and red arrows) and depletion wavelengths (dark red arrow). (D) Excitation (blue, λem = 532− 633 nm) and emission (red, λex = 465 nm) spectrum of NaYF4:Eu3+ (3%) nanoparticles. The sample was prepared by letting a droplet of nanoparticle solution dry out in order to create a dense particle film.

depletion laser (STED beam profile) for over 4 min, which did not result in any changes of the luminescence intensity. NaYF4:Eu3+ Nanoparticles. Replacing Dy3+ with Eu3+ in the synthesis (see Scheme 1) yielded NaYF4:Eu3+ (3%) nanoparticles. TEM images in Figure 3A and the size histogram in Figure 3B show that the particles are smaller in comparison to the NaYF4:Dy3+ (3%) nanoparticles and have an average diameter of 15.8 ± 1.3 nm. Figure 3C and 3D show the relevant part of the energy-state diagram together with the excitation and emission spectrum of Eu3+.17 In the case of Eu3+, direct excitation is possible at 465 nm. Upon direct excitation at 465 nm, Eu3+ will transfer from the 7F0 ground state to the 5 D2 excited state, followed by nonradiative transitions to 5D0. From the 5D0 state, emission will occur at 579 nm (5D0 to 7F0), 590 nm (5D0 to 7F1), 615 nm (5D0 to 7F2), and 695 nm (5D0 to 7 F4). We chose to deplete the 5D0 state via the 695 nm transition (5D0 to 7F4). It is worth mentioning that also radiative transitions occur from the 5D2 and 5D1 states. These states are also partially depleted by the 695 nm depletion beam, as can be seen in Figure S5; however, this is not as efficient as the targeted 5D0 state. Hence, emission originating from the 5D2 and 5D1 states should be excluded in the STED detection window. Additionally, the depletion beam increases the emission in the 564−574 nm range, most likely because of an excited-state absorption process. The latter seems to have little to no impact on the overall depletion of the 5D0 state. The depletion efficiency of the 5D0 state was checked by overlapping diffraction-limited excitation (465 nm) and depletion (695 nm) spots and monitoring the decrease in luminescence upon increasing the depletion intensity for a single nanoparticle. Figure 4A shows a single NaYF4:Eu3+ (3%) nanoparticle intensity trace with periodically blocked and unblocked depletion laser illumination. (A similar experiment was also performed for Eu3+ ions in D2O, which can be found in Figure S1.) Measuring the remaining emission at different

was proven by measuring the 572 nm emission region using only 748 nm illumination. Figure S3 shows the upconversion emission related to the 4F9/2 to 6H13/2 transition together with the power dependence. The latter exhibits a slope close to 1 (0.91) on a log−log scale. Such a linear power dependence can be explained by going through an intermediate state (e.g., 6 F3/2) and high excitation intensities.33,34 Now that we demonstrated that efficient depletion of more than 90% is possible, we changed the beam profile of the depletion laser from a diffraction-limited spot to the typical doughnut-shaped beam profile.6 Panels C and D of Figure 2 demonstrate that upon changing from confocal imaging (Figure 2C) to STED imaging (Figure 2D), the optical resolution of the individual NaYF4:Dy3+ (3%) nanoparticles is significantly improved. Panels E and F of Figure 2 show two cross sections taken from the confocal and STED images. Fitting the peaks of these cross sections with a Lorentzian35 function demonstrates that a fwhm of 90 nm can be obtained at 320 MW/cm2 depletion intensity. Additionally, we also tried the 454.6 nm line from a CW argon ion laser as excitation source for the NaYF4:Dy3+ (3%) nanoparticles. For this, a different region was scanned both confocal and in STED mode, increasing the integration time per pixel from 1 to 10 ms. These results can be seen in Figure 2G,H and demonstrate clearly that STED can separate close-lying NaYF4:Dy3+ (3%) nanoparticles which were undistinguishable before. Panels G and H of Figure 2 also illustrate that scanning the same area confocal and in STED mode is limited by scanner stability and sample drift. Furthermore, long time exposure of the nanoparticles in both confocal and STED modes demonstrates the well-known photostability of the inorganic lanthanide nanoparticles.18,36 Figure S4 shows the continuous illumination of a single NaYF4:Dy3+ (3%) nanoparticle with 449, 452, and 473 nm only and in combination with the 748 nm D


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Figure 4. (A) Intensity time trace of the emission of a single NaYF4:Eu3+ (3%) nanoparticle, without and with 695 nm coillumination. The excitation intensity was 670 kW/cm2; the depletion intensity was 222 MW/cm2, and the bin time was 50 ms. From the high and low intensity levels the intensity ratio in panel B was calculated. (B) Dependence of intensity ratios as a function of the depletion laser intensity. The line represents a 1/(1 + I/Is) fit to the data. (C) Confocal image of single NaYF4:Eu3+ (3%) nanoparticles spin coated from solution on a glass coverslip. (D) STED image of the same region as panel C. The particles were excited at 465 nm (670 kW/cm2) and depleted at 695 nm (222 MW/cm2). The pixel dwell time is 2 ms for both images. (E and F) Cross section profiles of the dashed line 1 in panels C and D. Confocal data is represented in blue, while STED data is plotted in red. The numbers give the fwhm value for a Gaussian fit (confocal) or a Lorentzian fit (STED), respectively.

doped NaYF4 nanoparticles as proof of concept. The Dy3+- and Eu3+-doped NaYF4 nanoparticles feature unmatched photostability compared to organic fluorophores and have similar IS values. Our findings suggest future applications where also other lanthanides (e.g., samarium, holmium, and erbium) can create similar STED labels embedded in the same or a range of different surface functionalized nanoparticles for specific target recognition. The myriad of excitation, emission, and depletion wavelengths available might also pave the way for future use of single lanthanide ion complexes, given they contain a photostable sensitizing antenna, as STED nanoscopy labels.

depletion intensities allowed the construction of the intensity ratio shown in Figure 4B. In agreement with the observed behavior for Dy3+, Eu3+ follows again the 1/(1 + I/Is) dependency as a function of the depletion intensity.31 With an average decay time for the 5D0 state of 5.7 ms (see Figure S2), the CW depletion behavior is again applicable for Eu3+. An IS value of 3.3 MW/cm2 was achieved for the NaYF4:Eu3+ (3%) nanoparticles, which is in the same range as the IS value of the NaYF4:Dy3+ (3%) nanoparticles. This allowed us to achieve depletion efficiencies of 96% at 222 MW/cm2. We changed the beam profile of the depletion laser from a diffraction-limited spot to a doughnut-shaped profile, to check the STED performance of the NaYF4:Eu3+ (3%) nanoparticles.6 Panels C and D of Figure 4 verify that when changing from confocal imaging with 465 nm only (Figure 4C) to STED imaging (Figure 4D), the optical resolution is increased. Panels E and F of Figure 4 show two cross sections of the confocal and STED images. Fitting the peaks of these cross sections with a Lorentzian35 function demonstrates that an fwhm of 130 nm can be obtained.

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MATERIALS AND METHODS

Nanoparticle Synthesis. NaYF4:Dy3+ (XDy = 0.03) and NaYF4:Eu3+ (XEu = 0.03) nanoparticles were synthesized in organic oils.30 XLn is defined as the mole fraction of Ln versus Y + Ln of the initial mixture at the start of the synthesis. The nanoparticles were rendered water dispersible by first removing the oleic acid surface with acid treatment and then coating the nanoparticles with poly(acrylic acid) (PAA, M = 2000 g mol−1).37 STED Setup. The experimental setup is schematically depicted in Figure S6. Images, time traces, and spectra were acquired with a home-built confocal microscope.38 A continuum white light laser with an acousto-optic tunable filter (NKT photonics) provided the excitation light for dysprosium (449, 452, and 473 nm) and europium (465 nm). The light was coupled into an optical fiber (NKT photonics) to achieve a Gaussian beam profile. Alternatively, the 454.6 or 465.8 nm lines of a CW Argon ion laser (CVI Melles-Griot

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CONCLUSION In this work, lanthanide-based optical nanoscopy has been demonstrated for STED imaging applications. We have demonstrated our general approach, using direct excitation without the need of a sensitizer, by using Dy3+- and Eu3+E



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35MAP431-200) were used in some experiments. After the output coupler of the fiber, the excitation beam was extended by a telecentric lens system (10 and 100 mm). A 500 nm (Thorlabs, FES0500) shortpass filter cleaned up residuals in the visible range. The depletion light for dysprosium (748 nm) and europium (695 nm) was provided by a mode-locked picosecond Ti:sapphire laser (Tsunami, Spectra Physics). The beam was first cleaned up by either a tunable longpass filter (Versachrome TLP01-790, Semrock, for dysprosium) or a 647 nm long-pass filter (BLP01-647R, Semrock, for europium). The beam was sent into a 10 m polarization-maintaining single mode fiber (Thorlabs) producing a Gaussian beam profile. After the fiber output, the depletion beam was collimated by a 30 mm lens (Thorlabs) and the polarization was cleaned up by a polarizing beam splitter cube (Thorlabs). Afterward, the beam was steered into a 2π vortex phase plate (VPP-1a, RPC Photonics). The polarization of the resulting vortex beam was set to circular by a combination of achromatic λ/2 and λ/4 wave plates (Thorlabs). Excitation and depletion beams were coaligned by transmission/reflection through/from a 488 nm dichroic mirror (Di01-R488/561, Semrock) and then reflected by a dichroic mirror (TLP01-561 Versachrome, Semrock for dysprosium or 490CDLP, Omega for europium) into an oil immersion objective (Olympus, UPlanSApo 100×, NA = 1.4). The objective focused the beams onto the sample and collected the luminescence signal. Overlap of the two beams and optimization of the doughnut mode profile was achieved by scanning either 40 nm Fluospheres (660/680, Thermo Scientific) or 200 nm TetraSpeck beads (blue, green, orange, and dark red; Thermo Scientific), both giving excellent Stokes and anti-Stokes emission signal (see Figure S7). The sample was scanned with a piezo scanner (Physik Instrumente). Primary and secondary laser light was blocked by 561 nm (Semrock Edge Basic), 532 nm (Semrock Edge Basic) long-pass filters and 700 nm (Chroma, ET700SP-2P8), 633 nm (Semrock Edge Basic) short-pass filters in the detection path. For imaging and single-photon counting purposes, the luminescence signal was detected by an avalanche photodiode (PerkinElmer CD3226) connected to a single-photon counting module (Becker & Hickl SPC-830). Spectra were recorded by inserting a mirror in the detection path and reflecting the luminescence signal into a liquid nitrogen-cooled spectrograph (Princeton Instruments SPEC-10:100B/LN_eXcelon CCD camera, SP 2356 spectrometer, 300 grooves/mm). Luminescence lifetimes in the millisecond range were measured by modulating the excitation light with a chopper wheel (Stanford Research Systems) at 60 Hz (for dysprosium) or 10 Hz (for europium) while acquiring photon macrotimes. The macrotimes were then combined to form the luminescence decay histogram. Data were analyzed with self-written Matlab algorithms. The depletion curves in Figures 2B and 4B were fitted by applying a variance weighting (∼y2) to the fit.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the “Center for Synthetic Biology” at Copenhagen University funded by the UNIK research initiative of the Danish Ministry of Science, Technology and Innovation (Grant 09-065274), bioSYNergy, University of Copenhagen’s Excellence Programme for Interdisciplinary Research, the Villum Foundation (Project Number VKR023115), the Carlsberg Foundation (CF140388), and the Danish Council of Independent Research (Project number DFF-7014-00027). TEM images were taken in the Laboratory of Electron Microscopy at the University of Turku.

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REFERENCES

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.9b01272.

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NaYF4:Dy3+ (3%) and NaYF4:Eu3+ (3%) decay times, spectra, upconversion, and photostability data; experimental setup and alignment details (PDF)

AUTHOR INFORMATION

Corresponding Authors

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Satu Lahtinen: 0000-0003-2816-7809 Tom Vosch: 0000-0001-5435-2181 Notes

The authors declare no competing financial interest. F


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Lanthanide-Doped Nanoparticles for Stimulated Emission Depletion

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