Environmental Impact on the Excitation Path of the Red Upconversion

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Article

Environmental Impact on the Excitation Path of the Red Upconversion Emission of Nanocrystalline NaYF:Yb ,Er 4

3+

3+

Iko Hyppänen, Niina Höysniemi, Riikka Arppe, Michael Schaeferling, and Tero Soukka J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b01019 • Publication Date (Web): 07 Mar 2017 Downloaded from http://pubs.acs.org on March 13, 2017

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The Journal of Physical Chemistry

Environmental Impact on the Excitation Path of the Red Upconversion Emission of Nanocrystalline NaYF4:Yb3+,Er3+ Iko Hyppänen*,†, Niina Höysniemi‡, Riikka Arppe‡1, Michael Schäferling§, Tero Soukka‡ †

‡

Department of Chemistry, University of Turku, Vatselankatu 2, 20014 Turku, Finland

Department of Biochemistry/Biotechnology, University of Turku, Tykistökatu 6 A, 20520 Turku, Finland

§

BAM Federal Institute for Materials Research and Testing, Division 1.10 – Biophotonics, Richard-Willstätter-Str. 11, D-12489 Berlin, Germany

Corresponding Author *[email protected]

1

Current address: Nano-Science Center & Department of Chemistry, University of Copenhagen, Universitetsparken 5, 2100 København Ø, Denmark

1 Environment ACS Paragon Plus


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ABSTRACT

The mechanism for red upconversion luminescence of Yb-Er codoped materials is not generally agreed in the literature. Both two-photon and three-photon processes have been suggested as the main path for red upconversion emission. We have studied β-NaYF4:Yb3+,Er3+ nanoparticles in H2O and D2O and we propose that the nanoparticle environment is a major factor in the selection of the preferred red upconversion excitation pathway. In H2O efficient multiphonon relaxation (MPR) promotes two-photon mechanism through green emitting states, while in D2O MPR is less effective and the three-photon path involving back energy transfer to Yb3+ is the dominant mechanism. For the green upconversion emission our results suggest the common two-photon path through 4F9/2 energy state in both H2O and D2O.


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INTRODUCTION The enthusiasm concerning lanthanide-doped photon upconverting nanoparticles (UCNP) stems from their application prospects, which include bioimaging,1,2 pH and ion sensors,3,4,5 wavelength conversion in solar cells,6,7 and security technology.8,9 The most studied photon upconversion system is Yb–Er codoped materials. Under near-infrared (NIR) excitation Yb3+ ions act as sensitizer which absorb the excitation energy (2F7/2 → 2F5/2). The absorbed energy is transferred to Er3+ activators in sequential energy transfer upconversion (ETU) steps between Yb3+ and Er3+ ions.10 As a result the Er3+ ions are excited to high energy states and return to ground state by emission at visible wavelengths: green (ca. 525 nm, 2H11/2 → 4I15/2 and 540 nm,4S3/2 → 4I15/2), red (650 nm, 4F9/2 → 4I15/2) and blue (410 nm, 2H9/2 → 4I15/2). Especially in biological applications the red Er3+ emission is desired due to its penetration depth in biological matrix.11 The generally accepted two-photon upconversion path for the green emission of Er3+ in Yb–Er codoped materials is presented in Figure 1. The green emitting states (2H11/2/4S3/2) of Er3+ are populated by two successive ETU steps (4I15/2 → 4I11/2, 4I11/2 → 4F7/2) from neighboring Yb3+ ions in the host followed by nonradiative multiphonon relaxation (MPR) of Er3+ (4F7/2 → 2H11/2/4S3/2). Several different excitation paths for the population of the red emitting state (4F9/2) have been proposed.12,13,14 The suggested excitation paths involve energy transfer, multiphonon relaxation, back-transfer to Yb3+, direct ground state absorption in Er3+, and cross-relaxation processes. The factors influencing the emission brightness and red-to-green emission ratio include the host material, concentration of lanthanide ions, nanoparticle size, and excitation power density and pulse duration.15 Hence, it is impossible to give a universal answer for the excitation path of the



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red upconversion processes in Yb–Er codoped materials. Here, we will focus on the red emission of one of the most efficient photon upconverting materials, the hexagonal phase βNaYF4:Yb3+,Er3+, and introduce yet another influencing factor, the environment. Two most commonly proposed two-photon paths for the red emission of β-NaYF4:Yb3+,Er3+, are presented in Figure 1. In Path 1 two ETU steps to green emitting states of Er3+ (2H11/2/4S3/2) followed by MPR from the green emitting states to red emitting state (4F9/2). In Path 2 after first ETU step MPR occurs between intermediate energy states (4I11/2 → 4I13/2) and the second energy transfer step (4I13/2 → 4F9/2) populates the red emitting state. Jung et al. studied the preference of these two-photon paths for the red emission of β-NaYF4:Yb3+,Er3+/NaYF4 in n-hexane and suggested that the preferred path is the one involving MPR of the intermediate state 4I11/2.16 Recently, a three-photon path for the red emission of Er3+ was proposed by Anderson et al. (Path 3 in Figure 1).17 Subsequently Berry and May proposed the three-photon process to be the predominant path for red emission in micrometer-scale powder β-NaYF4:Yb3+,Er3+ material.13 The path begins with the two-photon excitation of the green emitting states. A third ETU step promotes the Er3+ ion to a dense manifold of states (4G/2K). After relaxing within the 4G/2K manifold, back energy transfer (BET) to Yb3+ can result in population of the red emitting state (4F9/2). Competing with the BET is MPR to the blue emitting state (2H9/2). These studies, however, were only done with dry powder samples or in non-polar solvent. Here, we have studied the green and red emissions of β-NaYF4:Yb3+,Er3+ nanocrystals (29-37 nm) in D2O and H2O, which is the solvent needed for all biological applications. In H2O strong quenching by OH-vibrations promote MPR, whereas D2O is similar to aprotic organic solvents with low quenching efficiency. Our conclusion is that the excitation path for red emission depends on the environment explaining the earlier partly conflicting observations. In H2O the red emitting state


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is predominantly populated via two-photon path (Path 1 in Figure 1) and in D2O the threephoton path (Path 3) is the principal route. We propose that the OH-vibration dependent preference of two- and three-photon paths also explains the reported dependence of red-to-green ratio on the volume percentage of water in ethanol/water mixture.18

Figure 1. Energy level diagram of NaYF4:Yb3+,Er3+ with red (~655 nm), green (~525 and ~540 nm) and blue (~410 nm) upconversion emissions and their proposed excitation paths. Solid, dotted, dashed and wavy arrows represent photon absorption or emission, energy transfer, back energy transfer and relaxation processes, respectively. Numbers 1, 2 and 3 mark the different excitation paths leading to the red emitting state.



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EXPERIMENTAL SECTION Material preparation and characterization NaYF4:Yb3+,Er3+ (xYb = 0.17, xEr = 0.03) nanophosphors were synthesized in octadecene/oleic acid.19 Bare UCNPs were prepared by removing the oleic acid (OA, technical grade, 90% purity, Sigma-Aldrich, St. Louis, MO) capping using a modified method of Bogdan et al.20 Finally UCNPs were dispersed into ultrapure mQ-H2O or D2O (99.9% purity, Sigma-Aldrich, St. Louis, MO) and stored in rotation at room temperature. Details of synthesis and surface modification have been described earlier.21,22 UCNP size distribution was measured with transmission electron microscopy and the crystal structure was studied with X-ray powder diﬀraction. The NaYF4:Yb3+,Er3+ were rod-shaped with dimensions of 29-37 nm and the crystal structure was pure hexagonal.23 The results indicating the OA-capping removal (FR-IR spectra) have been reported earlier.21 Luminescence spectrum measurements Upconversion emission spectra were measured at a UCNP-concentration of 10 mg ml−1 with optical fiber spectrometer a PC2000-CCD (Ocean Optics, Inc., Dunedin, FL). A laser diode driver 5060 (Newport, Irvine, CA) was used to control a fiber-coupled NIR laser diode IFC-975008 (Optical Fiber Systems, Inc., Chelmsford, MA) providing ca. 800 mW at 973 nm. A Peltier module controlled by a temperature controller WTC3243 (Wavelength Electronics, Inc., Bozeman, MT) was used to cool the laser diode and tune the laser wavelength.23 The optical part of the luminometer consisted of tubular excitation and emission chambers (Thorlabs, Inc., Newton, NJ) in a right-angle configuration. The sample chamber was an aluminum cube


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equipped with a Peltier element for thermoelectric cooling/heating and a 5 mm quartz NMR tube for samples. In the excitation light path, a longpass filter with a cutoff at 850 nm (RG850, Edmund Optics, Barrington, NJ) was used to ensure a pure NIR excitation. In the emission light path, shortpass filters with a cutoff at 850 nm or 800 nm (Extented Hot Mirror, Edmund Optics #46-386 or 10SWF-800-B, Newport, respectively) with a good transmission at visible wavelengths were used to exclude the scattered excitation radiation. Emission light was focused into an optical fiber connected to the spectrometer. The OOIIrrad software were used to record the upconversion emission spectra. The spectral response of the PC2000-CCD spectrometer was calibrated with a tungsten halogen light source LS-1-CAL (Ocean Optics Inc.). The optical power meter PM100D and sensor S310C (Thorlabs) were used to measure the NIR laser optical power. The laser beam diameter was estimated to be ∼3 mm when focused on samples. The downshifted excitation and emission spectra were measured with a Varian Cary Eclipse spectrofluorometer. Spectra were recorded at phosphorescence mode with a 0.1 ms delay and a 0.5 ms gate time. The UCNP concentration was 1.0 or 1.5 mg ml-1 in H2O or D2O. In order to compare spectral responses of PC2000-CCD and Varian spectrometers the upconversion spectra were measured also with Varian Cary Eclipse equipped with a 980 nm laser diode module C2021-F1 (Roithner Lasertechnik, Vienna, Austria) aligned to the excitation light path. Spectra were recorded at bio/chemiluminescence mode with a 180 ms gate time. A shortpass filter with a cutoff at 775 nm (775FW82-50S; Andover, Salem ,NH) was used to exclude excitation radiation entering the emission monochromator of the spectrometer. UCNP-concentration was 1.5 mg ml-1.



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Luminescence lifetime measurements Upconversion luminescence lifetimes were measured using a modular luminometer with pulsed excitation. An analog-to-digital converter NI USB-6251 (National Instruments, Austin, TX) was used to generate square-wave excitation pulse profile for the laser driver 5060 (Newport). The excitation source was a fiber-coupled NIR laser diode IFC-975-008 (Optical Fiber Systems). The optical part of the system was the same tubular chamber setup used in the spectral measurements. The emission wavelength was selected with interference filters with a suitable bandpass, 544/10 nm, or 650/10 nm (Thorlabs). The detector at the end of emission chamber was a head-on R1464 photomultiplier (Hamamatsu Photonics, Hamamatsu City, Japan). The photomultiplier signal was amplified in a high-speed current amplifier DHPCA-100 (Femto Messtechnik GmbH, Germany). The amplified signal was recorded with the NI USB-6251 A/D converter, which was connected to a computer via USB and controlled with a computer program written in LabVIEW 8.5 (National Instruments). The default length of the excitation pulse was 20 ms with total cycle time of 50 ms (40% duty cycle). During a single measurement the pulse profile was cycled 5 000 times resulting a measurement time of ca. 4 minutes. UCNP-concentration was 10 mg ml-1. Luminescence decay profiles were analyzed with the exponential decay fitting with Origin 2015 (Originlab Corporation, Northampton, MA). RESULTS AND DISCUSSION Upconversion luminescence spectra of NaYF4:Yb3+,Er3+ 10 mg ml-1 in H2O and D2O were measured versus the excitation power at 293 K. The excitation power density range used in measurements was 3.2–11.1 W cm-2. The normalized emission spectra are presented in Figure 2. In the spectra two emission bands at green wavelength (ca. 525 and 540 nm) and one at red


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wavelength (ca. 650 nm) can be observed. These bands are associated with 2H11/2 → 4I15/2, 4

S3/2 → 4I15/2, and 4F9/2 → 4I15/2 transitions of Er3+, respectively (Figure 1). The main difference

between H2O and D2O samples is clearly visible in the normalized spectra. In H2O the red-togreen luminescence intensity ratio is large (2.2) and unaffected by the excitation power (Figure 3a). In D2O the red-to-green ratio is smaller and increased by increasing excitation power (0.8– 1.2). The absolute emission intensity in D2O was fivefold compared to H2O (Figure S1 in the Supporting Information (SI)). This is contributed to effective quenching of Yb3+ excited state in water affecting upconversion efficiency in total.21

2.0

1.1

1.6 1.4 1.2

1.0

Excitation power -2 density (W cm ) 3.2 5.2 7.1 9.1 11.1

1000

b)

Excitation power -2 density (W cm ) 3.2 5.2 7.1 9.1 11.1

0.9

1.0 0.8 0.6

0.8 0.7 0.6

Luminescence Intensity

a)

Normalized Intensity

1.8

Normalized Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.5 0.4 0.3 0.2

0.4

c)

100

Slope H2O, Green 2.14 H2O, Red

0.1

0.2

10 0.0

0.0 510

540

570

600

630

660

Wavelength (nm)

690

-0.1

2.13

D2O, Green 1.89 D2O, Red

510

540

570

600

630

660

690

Wavelength (nm)

4

6

8

2.27

10

12

-2

Excitation power (W cm )

Figure 2. Normalized upconversion luminescence spectra of NaYF4:Yb3+,Er3+ 10 mg ml-1 in a) H2O and b) D2O in increasing excitation power density. The spectra are normalized at 541 nm. c) Intensities of green (525 and 540 nm) and red (650 nm) upconversion emission bands of NaYF4:Yb3+,Er3+ 10 mg ml-1 in H2O and D2O plotted versus excitation power in a log-log scale.

In Figure 2c the intensities of the green and red upconversion emission bands of NaYF4:Yb3+,Er3+ in H2O and D2O are plotted against the excitation power density. In the log-log



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plot (Figure 2c) the responses are linear within the studied excitation power density range. In H2O there is no difference between the slopes of the green (2.14) and red (2.13) emissions and they are close to the theoretical two-photon process slope (2). In D2O the slope of green emission (1.89) is slightly below 2. But the slope of red emission (2.27) in D2O is notably larger. Based on the presented results (Figure 2) we concluded that in H2O both green and red emissions follow a two-photon excitation path. We propose that the path involving MPR from green states 2

H11/2/4S3/2 → 4F9/2 (Path 1 in Figure 1) is the dominant route to the red emitting state in H2O.

The fact that in H2O the downshifted red emission is observed with excitation at both 488 nm (4I15/2 → 4F7/2) and 520 nm (4I15/2 → 2H11/2) (Figure S2 in the SI) demonstrates that the red emitting state can be populated by OH-vibration meditated MPR from green emitting levels. It can be argued that the direct excitation of green emitting states samples more the near-surface ions, where the upconversion is less efficient, and would thus be more prone to MPR, but, our results suggested that at least in the used UCNPs with diameter below 40 nm this was not the case. The red-to-green ratio in H2O under NIR was 2.2 (Figure 3a) and when the instrumental effects were compensated the observed red-to-green ratio under 488 nm was the same 2.2. This is a strong indication that the MPR from green states is the main path to the red upconversion emission within the entire nanoparticle volume. Furthermore, in H2O the red-to-green ratio is independent of excitation power under NIR, which suggest that they are populated via the same route.16


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3.0 2.4

a)

2.8

Red-to-Green Emission Ratio

Red-to-Green Emission Ratio

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.2 2.0

H2O

1.8

D2O

1.6 1.4 1.2 1.0

2.6

b) H2O D2O

2.4 2.2 2.0 1.8 1.6 1.4

0.8 1.2 0.6 2

4

6

8

10

12 -2

Excitation power (W cm )

1.0 270 280 290 300 310 320 330 340 350 360

Temperature (K)

Figure 3. The red-to-green upconversion emission intensity ratios of NaYF4:Yb3+,Er3+ 10 mg ml-1 in H2O and D2O plotted versus a) excitation power, b) sample temperature. Red is an integrated intensity of 650 nm emission and green is a total integrated emission of both 525 nm and 541 nm emissions.

The larger slope in D2O (Figure 2c) suggests that the red state is populated by three-photon excitation process. A three-photon process involving excitation to 4G/2K manifold of Er3+ followed by Er → Yb energy back transfer (Path 3 in Figure 1) has been proposed by Berry et al. for solid NaYF4:Yb3+,Er3+ powder.13,17 We suggest that the three-photon path is effective also in D2O. The deviation between the slope of red emission in D2O (2.27) and the slope of theoretical three-photon process (3) may be because of a partial reuse of the back transfer excited Yb3+ in ETU. Berry et al. further stated, that the three-photon process is a sole path for red emission in solid powder phase. They presented downshifted excitation spectra for red emission, where significant red emission was not produced by direct excitation to green emitting levels of 11 Environment ACS Paragon Plus


Er3+, but only with excitation below 400 nm, i.e. through excitation into 4G/2K manifold of Er3+.13 In D2O we observed the same for NaYF4:Yb3+,Er3+ in suspension, but in H2O the feeding of red emission clearly occurs also through green emitting states (Figure 4). The same conclusion can be made also from downshifted emission spectra of NaYF4:Yb3+,Er3+ in H2O and D2O (Figure S2 in the SI).

70

100

a)

Detection at 540 nm H2O

60

b) 2

H11/2

Intensity (RLU)

50 4

2

G/ K

40

30 4

20

4

Detection at 658 nm H2O

2

G/ K

80

D2O

Intensity (RLU)

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F7/2

D2O

60

40

2

20 10

4 2

H9/2

F3/2,5/2

2

H9/2

4 4

F3/2,5/2

H11/2

F7/2

4

S3/2

0 0 330 360 390 420 450 480 510 540 570

Wavelength (nm)

330 360 390 420 450 480 510 540 570

Wavelength (nm)

Figure 4. Downshifted excitation spectra of NaYF4:Yb3+,Er3+ 1 mg ml-1 in H2O and D2O with detection of a) 540 nm and b) 658 nm emission. Intensity as relative luminescence units (RLU).

In H2O the red-to-green ratio of upconversion emission is unaffected by NIR excitation power density (Figure 3a), whereas in D2O the ratio increases with increasing excitation density, suggesting higher order photon process for the red emission compared to the green emission. There are two factors which inhibit the three-photon process in H2O. First, due to the effective MPR mediated by OH-vibration, the lifetimes of the green emitting states are significantly


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shorter in H2O than in D2O (Table 1) and there is less time for the third ETU step to occur. Another factor is that the excited state of Yb3+ (2F5/2) is effectively quenched by water,21 hence decreasing the probability of ETU. In D2O these factors are not effective and the three-photon path is more probable. Further, the energy gaps in the 4G/2K manifold populated by the proposed three-photon path are small and readily relaxed by OD-vibrations or host lattice phonon energy, whereas in the two-photon paths the larger gaps (2H11/2/4S3/2 → 4F9/2 or 4I11/2 → 4I13/2) require effective OH-vibrations. We studied also the effect of sample temperature on the upconversion luminescence of NaYF4:Yb3+,Er3+ nanoparticles. The temperature range was 278 – 353 K (5, 20, 40, 60 and 80 o

C). The absolute intensity of both green and red upconversion emission decreased as

temperature was increased due to thermal quenching (Figure S3 in the SI). Normalized upconversion emission spectra are shown in Figure S4 (in the SI). Between the two green emissions, at 525 nm and 541 nm, the relative intensity of the 525 nm emission increases with temperature in both solvents due to the thermal coupling of green energy states.24 In H2O the redto-green ratio increases as the temperature increases (Figure 3b). The increased ratio in H2O indicates temperature dependency of the OH-vibration mediated nonradiative relaxation in the path leading to the red emitting state. In D2O the red-to-green ratio stays constant with increased temperature. The OD-vibrations are less effective quenchers compared to OH-vibrations. However, the complete lack of temperature effect on red-to-green ratio in D2O further supports the hypothesis that in D2O the pathway to the red emitting state is not by MPR from the green emitting states.



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Upconversion process was studied also by decay measurements. Upconversion luminescence decay curves recorded at 544 nm and 650 nm are presented in Figure S5 (in the SI). The decay curves were analyzed with the exponential decay fitting (Equation 1) t

I = I B + ∑ Ai ⋅ e

τi

(1)

i

where I is the luminescence intensity at time t, IB is the baseline intensity of the decay curve, Ai and τi are the amplitude and decay time of ith component, respectively. Upconversion decay curves were fitted with three components (Table 1). It should be noted that the fitted decay times are not intrinsic luminescence decay rates of the emitting states, but apparent decay times of the detected emissions. They rise from complex excitation and emission system between Yb3+ and Er3+ and long-lived intermediate energy states. One fitting component has a negative amplitude indicating a luminescence rise time (τr) due to ETU excitation mechanism populating the emitting levels still after the excitation pulse. Other two components were denoted as emission decay times, τ1 and τ2. The shorter one (τ1) is the main component with a large relative amplitude (A1). At 544 nm the length of τ1 is almost doubled in D2O compared to H2O. Hence, this decay time originates from Er3+ ions susceptible to solvent quenchers. At 650 nm the difference in the length of τ1 is less dramatic, most likely because the red emitting state (4F9/2) is not readily quenched by OH-vibrations.21 The longer decay time (τ2) was associated with isolated Er3+ ions in the core of the nanoparticles, which are shielded from direct solvent quenching, and efficient sensitizer to sensitizer energy migration. The assignment is supported by the fact that the relative amplitude (A2) is larger in H2O, where the A1 population is effective quenched.


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Table 1. Fitting Results of Upconversion Luminescence Decay Curves of NaYF4:Yb3+,Er3+ 10 mg ml-1 in H2O and D2O. sample

wavelength (nm)

τra (µs)

τ1 (µs)

A1b (%)

τ2 (µs)

A2 (%)

H2O

544

23 ± 0.8

89 ± 0.4

86

616 ± 1.6

14

D2O

544

42 ± 0.5

171 ± 0.3

96

599 ± 4.2

4.2

H2O

650

59 ± 0.7

286 ± 0.7

85

688 ± 2.9

15

D2O

650

93 ± 0.4

294 ± 0.3

97

919±6.6

2.9

a

τi are decay times

b

Ai are amplitudes

CONCLUSIONS We have studied the upconversion mechanism of β-NaYF4:Yb3+,Er3+ nanoparticles in H2O and D2O. Based on the presented experimental results of upconversion and downshifted luminescence we propose that in both solvents the green emitting states of Er3+ are populated by generally agreed two-photon path. For the more disputed red emission path the evidence suggests that the separate two-photon and three-photon paths are primarily active in H2O and D2O, respectively. Our results indicate that in H2O the red emitting state is reached via MPR from green emitting states. In D2O we propose a three-photon excitation process to 4G/2K manifold and subsequent back energy transfer to Yb3+ resulting with Er3+ ion in red emitting state. However, when considering the other recent studies of the excitation paths of UCNPs, it is evident, that the path to red emitting state is sensitive not only to UCNP composition but also to the local environment. Understanding the mechanism of the upconversion process is essential in development of UCNPs and their applications, and the observed environmental effect should be taken into consideration when modelling the upconversion processes. Knowledge of the involved



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energy states is advantageous if additional dopants are used as energy acceptors or donors. Our results demonstrate that intended sample environment should be considered when designing UCNPs. Further, our results also suggest that red-to-green ratio or green emission lifetimes could be used to sensing water content in D2O or organic solvents. ASSOCIATED CONTENT Supporting Information Additional spectra and decay curves are available in the Supporting Information. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This study was supported by Tekes, the Finnish Funding Agency for Technology and Innovation. Michael Schäferling gratefully acknowledges financial support from DFG (Deutsche Forschungsgemeinschaft) for a Heisenberg research fellowship (SCHA 1009/10-1). The authors would like to acknowledge the COST Action CM1403 funded by the European Union. The authors would also like to thank Emilia Palo for synthesizing the UCNPs.


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REFERENCES (1) Liu, Q.; Feng, W.; Yang, T.; Yi, T.; Li, F. Upconversion Luminescence Imaging of Cells and Small Animals. Nat. Protoc. 2013, 8, 2033–2044. (2) Christ, S.; Schäferling, M. Chemical Sensing and Imaging Based on Photon Upconverting Nano- and Microcrystals: A Review. Methods Appl. Fluoresc. 2015, 3, 034004. (3) Arppe, R.; Näreoja, T.; Nylund, S.; Mattsson, L.; Koho, S.; Rosenholm, J. M.; Soukka, T.; Schäferling, M. Photon Upconversion Sensitized Nanoprobes for Sensing and Imaging of pH. Nanoscale 2014, 6, 6837–6843. (4) Liu, Q.; Peng, J.; Sun, L.; Li, F. High-Efficiency Upconversion Luminescent Sensing and Bioimaging of Hg(II) by Chromophoric Ruthenium Complex-Assembled Nanophosphors. ACS Nano 2011, 5, 8040–8048. (5) Wei, R.; Wei, Z.; Sun, L.; Zhang, J. Z.; Liu, J.; Ge, X.; Shi, L. Nile Red DerivativeModified Nanostructure for Upconversion Luminescence Sensing and Intracellular Detection of Fe3+ and MR Imaging. ACS Appl. Mater. Interfaces 2016, 8, 400–410. (6) de Wild, J.; Meijerink, A.; Rath, J. K.; van Sark, W. G. J. H. M.; Schropp, R. E. I. Upconverter Solar Cells: Materials and Applications. Energy Environ. Sci. 2011, 4, 4835–4848. (7) Fischer, S.; Favilla, E.; Tonelli, M.; Goldschmidt, J. C. Record Efficient Upconverter Solar Cell Devices with Optimized Bifacial Silicon Solar Cells and Monocrystalline BaY2F8:30% Er3+ Upconverter. Sol. Energy Mater. Sol. Cells 2015, 136, 127–134.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 21

(8) Meruga, J. M.; Cross, W. M.; May, P. S.; Luu, Q.; Crawford, G. A.; Kellar, J. J. Security Printing of Covert Quick Response Codes Using Upconverting Nanoparticle Inks. Nanotechnology 2012, 23, 395201. (9) Blumenthal, T.; Meruga, J.; May, P. S.; Kellar, J.; Cross, W.; Ankireddy, K.; Vunnam, S.; Luu, Q. N. Patterned Direct-Write and Screen-Printing of NIR-to-Visible Upconverting Inks for Security Applications. Nanotechnology 2012, 23, 185305. (10) Auzel, F. Upconversion and Anti-Stokes Processes with f and d Ions in Solids. Chem. Rev. 2004, 104, 139–173. (11) Kuningas, K.; Päkkila, H.; Ukonaho, T.; Rantanen, T.; Lövgren, T.; Soukka, T. Upconversion Fluorescence Enables Homogeneous Immunoassay in Whole Blood. Clin. Chem. 2007, 53, 145–146. (12) Auzel, F. E. Materials and Devices Using Double-Pumped Phosphors with Energy Transfer. Proc. IEEE 1973, 61, 758–786. (13) Berry, M. T.; May, P. S. Disputed Mechanism for NIR-to-Red Upconversion Luminescence in NaYF4:Yb3+,Er3+. J. Phys. Chem. A 2015, 119, 9805–9811. (14) Xu, D.; Liu, C.; Yan, J.; Yang, S.; Zhang, Y. Understanding Energy Transfer Mechanisms for Tunable Emission of Yb3+-Er3+ Codoped GdF3 Nanoparticles: ConcentrationDependent Luminescence by Near-Infrared and Violet Excitation. J. Phys. Chem. C 2015, 119, 6852–6860. (15) Nadort, A.; Zhao, J.; Goldys, E. M. Lanthanide Upconversion Luminescence at the Nanoscale: Fundamentals and Optical Properties. Nanoscale 2016, 8, 13099–13130.


Page 19 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(16) Jung, T.; Jo, H. L.; Nam, S. H.; Yoo, B.; Cho, Y.; Kim, J.; Kim, H. M.; Hyeon, T.; Suh, Y. D.; Lee, H.; et al. The Preferred Upconversion Pathway for the Red Emission of LanthanideDoped Upconverting Nanoparticles, NaYF4:Yb3+,Er3+. Phys. Chem. Chem. Phys. 2015, 17, 13201–13205. (17) Anderson, R. B.; Smith, S. J.; May, P. S.; Berry, M. T. Revisiting the NIR-to-Visible Upconversion Mechanism in β-NaYF4:Yb3+,Er3+. J. Phys. Chem. Lett. 2014, 5, 36–42. (18) Cong, T.; Ding, Y.; Xin, S.; Hong, X.; Zhang, H.; Liu, Y. Solvent-Induced Luminescence Variation of Upconversion Nanoparticles. Langmuir 2016, 32, 13200–13206. (19) Ylihärsilä, M.; Harju, E.; Arppe, R.; Hattara, L.; Hölsä, J.; Saviranta, P.; Soukka, T.; Waris, M. Genotyping of Clinically Relevant Human Adenoviruses by Array-in-Well Hybridization Assay. Clin. Microbiol. Infect. 2013, 19, 551–557. (20) Bogdan, N.; Vetrone, F.; Ozin, G. A.; Capobianco, J. A. Synthesis of Ligand-Free Colloidally Stable Water Dispersible Brightly Luminescent Lanthanide-Doped Upconverting Nanoparticles. Nano Lett. 2011, 11, 835–840. (21) Arppe, R.; Hyppänen, I.; Perälä, N.; Peltomaa, R.; Kaiser, M.; Würth, C.; Christ, S.; Resch-Genger, U.; Schäferling, M.; Soukka, T. Quenching of the Upconversion Luminescence of NaYF4:Yb3+,Er3+ and NaYF4:Yb3+,Tm3+ Nanophosphors by Water: The Role of the Sensitizer Yb3+ in Non-Radiative Relaxation. Nanoscale 2015, 7, 11746–11757. (22) Palo, E.; Tuomisto, M.; Hyppänen, I.; Swart, H. C.; Hölsä, J.; Soukka, T.; Lastusaari, M. Highly Uniform Up-Converting Nanoparticles: Why You Should Control Your Synthesis Even More. J. Lumin. 2017, 185, 125–131.



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(23) Hyppänen, I.; Perälä, N.; Arppe, R.; Schäferling, M.; Soukka, T. Environmental and Excitation Power Effects on the Ratiometric Upconversion Luminescence Based Temperature Sensing Using Nanocrystalline NaYF4:Yb3+,Er3+. ChemPhysChem 2017, in press, DOI: 10.1002/cphc.201601355. (24) Suo, H.; Guo, C.; Li, T. Broad-Scope Thermometry Based on Dual-Color Modulation upConversion Phosphor Ba5Gd8Zn4O21:Er3+/Yb3+. J. Phys. Chem. C 2016, 120, 2914–2924.


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TOC Graphic


Environmental Impact on the Excitation Path of the Red Upconversion

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