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Optical Materials Express

Optical Materials Express

  • Editor: David J. Hagan
  • Vol. 3, Iss. 5 — May. 1, 2013
  • pp: 566–573
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Rare-earth doped particles with tunable infrared emissions for biomedical imaging

Bryan van Saders, Lara Al-Baroudi, Mei Chee Tan, and Richard E. Riman  »View Author Affiliations


Optical Materials Express, Vol. 3, Issue 5, pp. 566-573 (2013)
http://dx.doi.org/10.1364/OME.3.000566


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Abstract

The tunability, brightness, and energy efficiencies of infrared-emitting rare earth doped nanomaterials are important performance parameters for biomedical imaging applications. In this work, hexagonal phase NaYF4:Yb3+, Ln3+ (Ho3+, Tm3+ and Pr3+) was synthesized and optimized using a facile hydrothermal method in the presence of poly(vinyl-pyrrolidone). Distinct infrared emission peaks were measured at 1185, 1310 and 1475 nm upon excitation at 980 nm. The optical efficiencies of NaYF4:Yb3+, Ln3+ at optimal concentrations were measured to quantify the brightness of these particles in comparison to that of NaYF4:Yb3+, Er3+ particles. Efficiencies were ranked as Er3+>Ho3+>Tm3+>Pr3+.

© 2013 OSA

1. Introduction

Phosphors with down-conversion emissions in the infrared have attracted attention due to their applications in lasing systems (wavelength, λ = 1000-3000 nm), optical amplifiers (λ = 1400-1600 nm), and photovoltaics (λ = 800-1100 nm) [1

1. A. Jha, B. Richards, G. Jose, T. Teddy-Fernandez, P. Joshi, X. Jiang, and J. Lousteau, “Rare-earth ion doped TeO2 and GeO2 glasses as laser materials,” Prog. Mater. Sci. 57(8), 1426–1491 (2012). [CrossRef]

4

4. K. Deng, T. Gong, L. Hu, X. Wei, Y. Chen, and M. Yin, “Efficient near-infrared quantum cutting in NaYF4: Ho3+, Yb3+ for solar photovoltaics,” Opt. Express 19(3), 1749–1754 (2011). [CrossRef] [PubMed]

]. In addition to these applications, infrared emissions in the range of 1200-1700 nm are of interest for biomedical imaging applications [5

5. A. M. Smith, M. C. Mancini, and S. Nie, “Bioimaging: second window for in vivo imaging,” Nat. Nanotechnol. 4(11), 710–711 (2009). [CrossRef] [PubMed]

]. The first (700-900 nm) and second (1200-1700 nm) windows are ranges of optical transparency in biological tissues due to reduced absorption and scattering [5

5. A. M. Smith, M. C. Mancini, and S. Nie, “Bioimaging: second window for in vivo imaging,” Nat. Nanotechnol. 4(11), 710–711 (2009). [CrossRef] [PubMed]

9

9. D. J. Naczynski, T. Andelman, D. Pal, S. Chen, R. E. Riman, C. M. Roth, and P. V. Moghe, “Albumin nanoshell encapsulation of near-infrared-excitable rare-Earth nanoparticles enhances biocompatibility and enables targeted cell imaging,” Small 6(15), 1631–1640 (2010). [CrossRef] [PubMed]

], allowing contrast agents which operate at these wavelengths to penetrate biological systems deeper on the scale of millimeters to centimeters. Infrared emissions for imaging in the 1100-1600 nm range have been reported to improve imaging sensitivity by up to ~10 times [10

10. M. C. Tan, G. A. Kumar, R. E. Riman, M. G. Brik, E. Brown, and U. Hommerich, “Synthesis and optical properties of infrared emitting YF3:Nd nanoparticles,” J. Appl. Phys. 106(6), 063118 (2009). [CrossRef]

12

12. A. L. Rogach, A. Eychmüller, S. G. Hickey, and S. V. Kershaw, “Infrared-emitting colloidal nanocrystals: synthesis, assembly, spectroscopy, and applications,” Small 3(4), 536–557 (2007). [CrossRef] [PubMed]

].

Rare earth doped materials can be tailored to exhibit absorption and emission lines within the first and second windows. The absorption and emission properties of rare earth doped phosphors can be tailored by controlling the local environment, such as site symmetry, crystal field strength and electron-phonon interaction strength of the rare-earth dopants. Tunable emissions in the second window can be achieved under the same excitation by selecting different rare earth ions as emitting centers, which will allow for multispectral imaging. Multispectral imaging is the use of various surface targeting ligands on two or more phosphors with non-overlapping emissions to identify different diseased sites and states. Figure 1
Fig. 1 Energy levels of Yb3+, Tm3+, Pr3+, Ho3+, and Er3+ showing excitation and emission pathways.
shows how Yb3+ can serve as a sensitizer to enhance 980 nm absorption and energy transfer to a number of rare earth emitting ions (Ho3+, Pr3+, Tm3+, Er3+) with infrared emissions within the second window. In phosphate and telluride glasses, it has been shown that Yb3+- Ln3+ (Ln3+ = Ho3+, Tm3+, and Pr3+) systems are capable of down-conversion of 980 nm excitation to 1200, 1450, and 1300 nm [13

13. S. Tanabe, T. Kouda, and T. Hanada, “Energy transfer and 1.3 µm emission in Pr-Yb codoped tellurite glasses,” J. Non-Cryst. Solids 274(1-3), 55–61 (2000). [CrossRef]

16

16. X. Zou and H. Toratani, “Dynamics and mechanics of up-conversion processes in Yb3+ sensitized Tm3+- and Ho3+-doped fluorozircoaluminate glasses,” J. Non-Cryst. Solids 181(1-2), 87–99 (1995). [CrossRef]

]. The 1550 nm emission characteristics and optical efficiency of another shortwave infrared emitting chemistry, NaYF4:Yb3+-Er3+, was previously reported and will be used as a reference in this work [17

17. M. C. Tan, J. Connolly, and R. E. Riman, “Optical efficiency of short wave infrared emitting phosphors,” J. Phys. Chem. C 115(36), 17952–17957 (2011). [CrossRef]

]. Halide hosts (e.g. NaYF4, YF3, LaF3, CaF2) are favored for their low phonon energies that minimize non-radiative losses to enable bright SWIR emissions. The use of hexagonal NaYF4 as a host material is advantageous due to its low phonon energy which promotes radiative relaxation, and favorable lattice sites for substitution with lanthanide ions [18

18. G. Wang, W. Qin, J. Zhang, L. Wang, G. Wei, P. Zhu, and R. Kim, “Controlled synthesis and luminescence properties from cubic to hexagonal NaYF4: Ln3+ (Ln=Eu and Yb/Tm) microcrystals,” J. Alloy. Comp. 475(1-2), 452–455 (2009). [CrossRef]

]. However, the widespread use of NaYF4 as a biomedical imaging marker is limited due to its potential cytotoxicity and lack of functionalized surface sites [9

9. D. J. Naczynski, T. Andelman, D. Pal, S. Chen, R. E. Riman, C. M. Roth, and P. V. Moghe, “Albumin nanoshell encapsulation of near-infrared-excitable rare-Earth nanoparticles enhances biocompatibility and enables targeted cell imaging,” Small 6(15), 1631–1640 (2010). [CrossRef] [PubMed]

]. Recent work has shown that the use of functionalized albumin coatings can mitigate these factors and yield contrast agents comparable to quantum dots in terms of functionality [9

9. D. J. Naczynski, T. Andelman, D. Pal, S. Chen, R. E. Riman, C. M. Roth, and P. V. Moghe, “Albumin nanoshell encapsulation of near-infrared-excitable rare-Earth nanoparticles enhances biocompatibility and enables targeted cell imaging,” Small 6(15), 1631–1640 (2010). [CrossRef] [PubMed]

], while displaying low systemic toxicity [19

19. J. C. Zhou, Z. L. Yang, W. Dong, R. J. Tang, L. D. Sun, and C. H. Yan, “Bioimaging and toxicity assessments of near-infrared upconversion luminescent NaYF4:Yb,Tm nanocrystals,” Biomaterials 32(34), 9059–9067 (2011). [CrossRef] [PubMed]

]. Besides the unique advantage of narrower emission bandwidths (<100 nm) and spectral tunability within both infrared windows, rare earth doped nanoparticles are also more stable than conventional organic dyes, showing almost no loss in emission intensities over months. In comparison, most organic dyes suffer from poor photostability, which results in reducing emission intensities after a day. Alternative infrared-emitting inorganic semiconductor substitutes comprise several well-known toxic elements (e.g. Hg, Cd and Pb), and are thus not favorable for biomedical applications, aside from their poor emission characteristics [5

5. A. M. Smith, M. C. Mancini, and S. Nie, “Bioimaging: second window for in vivo imaging,” Nat. Nanotechnol. 4(11), 710–711 (2009). [CrossRef] [PubMed]

,11

11. Y. T. Lim, S. Kim, A. Nakayama, N. E. Stott, M. G. Bawendi, and J. V. Frangioni, “Selection of quantum dot wavelengths for biomedical assays and imaging,” Mol. Imaging 2(1), 50–64 (2003). [CrossRef] [PubMed]

,12

12. A. L. Rogach, A. Eychmüller, S. G. Hickey, and S. V. Kershaw, “Infrared-emitting colloidal nanocrystals: synthesis, assembly, spectroscopy, and applications,” Small 3(4), 536–557 (2007). [CrossRef] [PubMed]

]. The detailed comparison and discussion of the rare earth doped materials, organic dyes and quantum dots is discussed elsewhere [20

20. D. J. Naczynski, M. C. Tan, M. Zevon, B. Wall, J. Kohl, A. Kulesa, S. Chen, C. M. Roth, R. E. Riman, and P. V. Moghe are preparing a manuscript to be called “Rare-earth doped biologic composites as shortwave infrared reporters in vivo”.

].

To date, the vast body of work concerning NaYF4: Yb3+, Ln3+ particle systems has focused on the up-conversion and down-conversion properties in the range of 300-1000 nm, with no evaluation of infrared down-conversion spectra in the range of 1000-1500 nm. In this work, we will examine the feasibility of using NaYF4 as a host for multispectral shortwave infrared emissions in the second “biologically transparent” window. A complete study on the application of these materials for biomedical imaging is described elsewhere [20

20. D. J. Naczynski, M. C. Tan, M. Zevon, B. Wall, J. Kohl, A. Kulesa, S. Chen, C. M. Roth, R. E. Riman, and P. V. Moghe are preparing a manuscript to be called “Rare-earth doped biologic composites as shortwave infrared reporters in vivo”.

]. Hexagonal microcrystals of NaYF4 at various dopant concentrations (i.e. Pr3+, Ho3+ and Tm3+) were synthesized to investigate the optimal doping concentrations for infrared emissions. Optical efficiencies were then measured to quantify their emissions brightness and ranked in comparison to a Kigre phosphate glass (QE-7S, Kigre Inc. Hilton Head, SC) and NaYF4: Yb3+, Er3+ as internal references [17

17. M. C. Tan, J. Connolly, and R. E. Riman, “Optical efficiency of short wave infrared emitting phosphors,” J. Phys. Chem. C 115(36), 17952–17957 (2011). [CrossRef]

]. Optical efficiency, unlike quantum efficiency, provides a practical brightness metric to evaluate the performance of these fluorophores for biomedical imaging [17

17. M. C. Tan, J. Connolly, and R. E. Riman, “Optical efficiency of short wave infrared emitting phosphors,” J. Phys. Chem. C 115(36), 17952–17957 (2011). [CrossRef]

].

2. Experimental methods

2.1 Hydrothermal synthesis of NaYF4: Yb3+, Ln3+ down-conversion particles

Powders with dopants concentrations corresponding to the Y3+: Ln3+ atomic ratios of ~(0.80 – x): x = (0.001, 0.0025, 0.005, 0.01, 0.02, 0.05, 0.1, 0.2, 0.3, 0.5) and constant Yb: (Y + Ln) atomic ratios at ~0.2: 0.80 were prepared. These powders were used to study the effects of varying the emitting ion concentrations on the emissions intensities. Using the optimized dopants concentration of each system (xopt), powders with concentrations corresponding to Ln: Yb atomic rations of xopt: (0.10, 0.15, 0.30, 0.35) were prepared to study the sensitizer concentration effects on emissions intensities.

Stoichiometric amounts of 99.8% yttrium (III) nitrate, 99.9% ytterbium (III) nitrate, 99.9 + % lanthanide (III) nitrate; where lanthanide = holmium, praseodymium, and thulium (Sigma Aldrich, St. Louis, Missouri), were mixed with 1.5 times excess 99.0% sodium fluoride (Sigma Aldrich), and 8.0 g of polyvinyl-pyrrolidone (average Mn ~40 000 g/mol from Sigma Aldrich) in ~70 mL water/ethanol mixture (80:20 v/v) for 30 min. The mixture was then transferred to a 125 mL TeflonTM liner, loaded in a Parr pressure vessel (Parr Instrument Co., Moline, IL) and heated to ~240°C for 4 hours. The as-synthesized particles were washed three times in ethanol followed by three times in deionized water by centrifuging (Beckman Coulter Avanti J-26 XP, Fullerton, CA) and dried at 70°C in a mechanical convection oven (Thermo Scientific Thermolyne, Waltham, MA) for powder characterization.

2.2 Powder characterization

Scanning electron microscopy (SEM) images of powder samples were taken using a Carl Zeiss Σigma field emission scanning electron microscope (Carl Zeiss, Carl Zeiss SMT Inc., Peabody, MA) using secondary electron detector operating at an accelerating voltage of 5.0 kV with a working distance of ~8.0 mm. Particle sizes, aspect ratios (i.e. major to minor axis), and morphology were evaluated manually from the SEM micrographs. Approximately thirty random particles were selected and measured for every powder sample from approximately 3 to 4 micrographs to provide the distribution of the particle sizes, and major to minor axis measurements. Energy dispersive X-ray (EDX) spectroscopy area scans of the powder samples were also completed to determine their elemental compositions under an accelerating voltage of 25 kV and a working distance of 8.5 mm for an aperture of 30 µm. The EDX elemental composition was evaluated by averaging elemental composition from five area scans and comparing the relative peak intensities assuming their total intensities to be 100%.

Powder X-ray diffraction (XRD) patterns were collected with a resolution of 0.04°/step size, 2 s/step using a Siemens D500 (Bruker AXS Inc., Madison, WI) powder diffractometer (40 kV, 30 mA) with Cu Kα radiation (λ = 1.54 Å). Powder diffraction file (PDF) from International Centre for Diffraction Data (Newton Square, PA) for hexagonal NaYF4 PDF #97-005-1917 was used as reference.

The powder samples were packed in demountable Spectrosil far UV quartz Type 20 cells (Starna Cells, Inc., Atascadero, CA) with 0.5 mm path lengths for emission collection. The emission spectra of the particles were collected using an Edinburgh Instruments FSP920 spectrometer (Edinburgh Instruments, Livingston, United Kingdom) equipped with a Hamamatsu R928P photomultiplier tube detector under an excitation of ~976 nm with an external 2.5W laser (BW976, B&W Tek, Newark, NJ).

~0.2 g of powder samples of the particles were dry pressed into pellets of 1 cm diameter and 2 mm thickness. These pellets were used to measure the optical efficiencies of the powders. A modification of the C9220-03 quantum yield measurement system (Hamamatsu, Bridgewater, NJ) was used to measure the optical efficiency. In brief, the measurement principle is based on direct illumination and indirect reflection. Light enters the integrating sphere through the sample port, goes through multiple reflections, and is scattered uniformly around the interior of the sphere. For our measurements, the integrating sphere was set up in reflectance mode to measure total integrated reflectance of a surface. The PD300-IR power detector (Ophir-Spiricon, Logan, UT) measured the power of emitted light was used in place of the photomultiplier tube that was originally on the C9220-03 quantum yield measurement system. It was positioned at the port on the side of the sphere were the emitted beam is independent of the angular properties of light at the sample port. A further assumption made during measurements is that all light emanating from the different samples is isotropic, since there is no preferential ordering of the powders during the consolidation to form pellets.

3. Results and discussion

The XRD profiles in Fig. 2
Fig. 2 XRD profile of NaYF4:Yb3+:RE3+ for RE3+ = Ho3+, Tm3+ and Pr3+. All diffraction peaks were attributed to hexagonal NaYF4.
show that pure hexagonal phase NaYF4: Yb3+, Ln3+ (Ln3+ = Ho3+, Tm3+, Pr3+) particles with different rare earth emitting ions (Ln3+) concentrations and different sensitizer (Yb3+) concentrations were synthesized and optimized using the hydrothermal method. Evaluation of the (100) diffraction peak located at 2θ = 17.2° using Scherrer’s equation [21

21. B. D. Cullity and S. R. Stock, Elements of X-ray Diffraction (Prentice Hall, 2001).

] showed average crystallite sizes of ~41 ± 7, 38 ± 5, and 33 ± 3 nm for Ho3+, Tm3+, and Pr3+ doped powders, respectively. The substitution of similarly sized rare earth dopants and the variation in concentration did not have an effect on observed XRD crystallite sizes. SEM images show that irregular, elongated micron-sized NaYF4: Yb3+, Ln3+ particles were prepared regardless of dopant chosen (Fig. 3
Fig. 3 SEM micro-graphs showing synthesized NaYF4: Yb0.2, Ln0.01, where Ln = (a) Ho3+, (b) Tm3+, and (c) Pr3+ doped NaYF4 particles. Distributions of major axis were 1.31 ± 0.49, 1.21 ± 0.51, and 1.25 ± 0.47 µm, respectively. Distributions of minor axis were 0.39 ± 0.10, 0.36 ± 0.11, 0.40 ± 0.12 µm, respectively.
). The major axis and minor axis of particle sizes were determined from the SEM micrographs. Since the crystallite sizes were significantly smaller than the particle sizes, it was concluded that polycrystalline phosphors were synthesized. No statistically significant differences in both particle and crystallite sizes at a 95% confidence level were observed between the different doping levels. All the particles synthesized had a broad size distribution. The major axes lengths ranged from ~0.3 to 4 µm while the minor axes lengths ranged from ~0.1 to 0.9 µm. The differences in crystallite sizes and particle sizes indicate that polycrystalline micron-sized particles of NaYF4:Yb3+, Ln3+ (Ln3+ = Ho3+,Tm3+,Pr3+) were synthesized.

The effect of Ho3+, Tm3+, and Pr3+ emitting ion concentrations on down-conversion emission brightness were investigated while keeping the sensitizer (Yb3+) concentration constant at 20 mol%. Distinct differences in the emission intensities of NaYF4: Yb3+, Ln3+ (Ln3+ = Ho3+, Tm3+, Pr3+) were observed. The integrated emission intensities for each system at different emitting ion concentrations are shown in Fig. 6
Fig. 6 Integrated emission intensity as a function of (a) Ho3+, Tm3+, or Pr3+ concentration in NaYF4, and (b) Yb3+ sensitizer concentration upon excitation at ~980 nm.
on a semi-logarithmic scale. Figure 6(a) shows the non-linear behavior of emission intensities with respect to the concentration of rare earth emitting ions. The profiles for the various emitting ions show similar parabolic shapes on the semi-log scale. As the concentration of Ho3+, Tm3+, or Pr3+ in the host lattice initially increased, the emission intensities increased to an optimal point (see Fig. 6(a)). With further concentration increases, the emission intensity for Tm3+ or Pr3+ doped particles declined, while Ho3+ doped particles showed a pronounced plateau in their emission intensities before declining too. The optimal concentration levels were found to be 1 mol% for Ho3+ and Tm3+ and 0.1 mol% for Pr3+. The parabolic behavior in emission intensities results from two competing factors as the Ln3+emitting ion concentration is varied. Below the optimal concentration, an increase in Ln3+ concentration increased the number of emitting ions, leading to increased emission intensities with the increase in concentration up until the optimal concentration. The further increase in concentration of Ln3+ emitting ion leads to concentration quenching [23

23. G. A. Hebbink, J. W. Stouwdam, D. N. Reinhoudt, and F. van Veggel, “Lanthanide(III)-doped nanoparticles that emit in the near-infrared,” Adv. Mater. 14(16), 1147–1150 (2002). [CrossRef]

,24

24. Y. S. Tver’yanovich, “Concentration quenching of luminescence of rare-earth ions in chalcogenide glasses,” Glass Phys. Chem. 29(2), 166–168 (2003). [CrossRef]

]. Concentration quenching is determined mainly by the dipole-dipole interaction between rare earth ions. The quenching effects vary according to R−6, where R is the interionic distance between emitting ions. The luminescence is completely quenched for ions separated at a distance shorter than R, whereas ions separated by a distance longer R is not subjected to complete quenching. The typical critical interionic distance where concentration quenching occurs is approximately between 0.5 to 2 nm for a range of different host systems [10

10. M. C. Tan, G. A. Kumar, R. E. Riman, M. G. Brik, E. Brown, and U. Hommerich, “Synthesis and optical properties of infrared emitting YF3:Nd nanoparticles,” J. Appl. Phys. 106(6), 063118 (2009). [CrossRef]

,23

23. G. A. Hebbink, J. W. Stouwdam, D. N. Reinhoudt, and F. van Veggel, “Lanthanide(III)-doped nanoparticles that emit in the near-infrared,” Adv. Mater. 14(16), 1147–1150 (2002). [CrossRef]

,24

24. Y. S. Tver’yanovich, “Concentration quenching of luminescence of rare-earth ions in chalcogenide glasses,” Glass Phys. Chem. 29(2), 166–168 (2003). [CrossRef]

]. The rare earth interionic distance of 5, 5 and 11 nm was estimated based on the optimum concentrations of Ho3+, Tm3+ and Pr3+ (i.e. 1.0, 1.0 and 0.1 mol%, respectively) observed at the maximum emission intensities in Fig. 6(a). These calculated rare earth interionic distances were found to be beyond the range where concentration quenching is typically significant [10

10. M. C. Tan, G. A. Kumar, R. E. Riman, M. G. Brik, E. Brown, and U. Hommerich, “Synthesis and optical properties of infrared emitting YF3:Nd nanoparticles,” J. Appl. Phys. 106(6), 063118 (2009). [CrossRef]

,23

23. G. A. Hebbink, J. W. Stouwdam, D. N. Reinhoudt, and F. van Veggel, “Lanthanide(III)-doped nanoparticles that emit in the near-infrared,” Adv. Mater. 14(16), 1147–1150 (2002). [CrossRef]

,24

24. Y. S. Tver’yanovich, “Concentration quenching of luminescence of rare-earth ions in chalcogenide glasses,” Glass Phys. Chem. 29(2), 166–168 (2003). [CrossRef]

].

Yb3+ (sensitizer)-concentration dependence was investigated in each system by holding the emitting ion concentration constant at levels that correspond to optimized emissions (1 mol% of Ho3+ and Tm3+, and 0.1 mol% of Pr3+). The integrated emission intensities of the particles at different sensitizer ion concentrations are shown in Fig. 6(b). The profiles show that emissions increase with increasing Yb3+ ions until it peaks at an optimal concentration before dropping off in a parabolic behavior. Results indicate that 20 mol% Yb3+ is the optimal concentration level regardless of emitting ion dopant in system. The behavior in emission intensities with variations in the sensitizer concentration is attributed to two competing factors that arise when the Yb3+-Ln3+ distances are varied. An increase in sensitizer concentration increases the number of Yb3+ ions and reduces the Yb3+-Ln3+ distances. Below the optimal concentration, this leads to enhanced energy transfer from Yb3+ to Ln3+ as the absorption of the 980 nm excitation was increased and transferred to the emitting ions. Successful energy transfer from sensitizer to emitting ion increases the likelihood of radiative transitions. At concentrations of Yb3+ greater than the optimal, additional Yb3+ ions reduced Yb3+ to Ln3+ transfer in favor of Yb3+ to Yb3+ transfer and relaxation via non-radiative pathways [16

16. X. Zou and H. Toratani, “Dynamics and mechanics of up-conversion processes in Yb3+ sensitized Tm3+- and Ho3+-doped fluorozircoaluminate glasses,” J. Non-Cryst. Solids 181(1-2), 87–99 (1995). [CrossRef]

,25

25. X. P. Chen, X. Y. Huang, and Q. Y. Zhang, “Concentration-dependent near-infrared quantum cutting in NaYF4: Pr3+, Yb3+ phosphor,” J. Appl. Phys. 106(6), 063518 (2009). [CrossRef]

].

To quantify and compare the performance of these down-conversion infrared-emitting particles, the optical efficiencies of their respective emissions were measured under the same excitation wavelength of ~980 nm. Table 1

Table 1. Optical Efficiencies of Various Samples upon Excitation at ~980 nm

table-icon
View This Table
shows the the optical efficiencies of the Ln3+ doped powders in dry pressed pellets form. Kigre Erbium phosphate glass QE-7S and NaYF4:Yb3+, Er3+ microparticles are included as an internal reference for the measurement of optical efficiencies [17

17. M. C. Tan, J. Connolly, and R. E. Riman, “Optical efficiency of short wave infrared emitting phosphors,” J. Phys. Chem. C 115(36), 17952–17957 (2011). [CrossRef]

]. The ranking of the down-conversion performance of these particles was NaYF4:Yb3+, Er3+ (1.17%) > Kigre (0.45%) > NaYF4:Yb3+, Ho3+ (0.021%) > NaYF4:Yb3+,Tm3+ (0.0080%) > NaYF4:Yb3+,Pr3+ (0.0017%). This ranking is consistent with the ranking of emission intensities discussed earlier (Fig. 5), where the ratios of the relative optical efficiencies are 688:265:12:5:1, respectively (Table 1). For the optical efficiency measurements, the emission intensities of the samples from all angles are collected using the integrating sphere. In contrast for measurements collected within the spectrometer, only the emission at one specific angle (i.e. 90° to incident excitation source) was measured. Therefore, the difference in the values for the ranking ratios obtained from Fig. 5 and Table 1 was due to the difference in accounting for sample emissions, where the optical efficiency reflects a more representative performance value as discussed in our other work [17

17. M. C. Tan, J. Connolly, and R. E. Riman, “Optical efficiency of short wave infrared emitting phosphors,” J. Phys. Chem. C 115(36), 17952–17957 (2011). [CrossRef]

]. The lower optical efficiency measured for Pr3+-, Ho3+- or Tm3+-doped systems compared with Er3+- doped systems (Fig. 1) can be attributed to several possibilities: density of energy levels, oscillatory strength, branching ratios, energy transfer rates and energy transfer or cross-relaxation pathways [22

22. J. Solé, L. Bausa, and D. Jaque, An Introduction to the Optical Spectroscopy of Inorganic Solids (John Wiley, 2005).

]. Further discussion regarding these differences is warranted when more detailed spectroscopy studies are performed in the future.

4. Conclusions

Polycrystalline hexagonal NaYF4:Yb3+,Ln3+ (Ln3+ = Ho3+, Tm3+, Pr3+) microparticles with down-conversion properties were synthesized and optimized using a facile hydrothermal process. Distinct emissions in the second infrared window were observed when excited at 980 nm so as to render them potentially useful in multispectral imaging. Dopants concentration levels were optimized to give the maximum possible emissions from these rare earth doped systems. The optimum concentration level of Pr3+ dopant was found to be 10 times less than those of Ho3+ and Tm3+ dopant concentrations. Efficiencies were ranked as Er3+>Ho3+>Tm3+>Pr3+. The Pr3+ system was found to be ~12 times less efficient than Ho3+ and ~5 times lower than Tm3+. For the actual application of these rare earth doped systems in biomedical imaging, further research to improve the optical efficiency of these particles to levels comparable to the NaYF4:Yb3+, Er3+ is warranted.

Acknowledgments

The authors thank the Defense Advanced Research Projects Agency (ONR-N00014-08-1-0131) for funding this research.

References and links

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10.

M. C. Tan, G. A. Kumar, R. E. Riman, M. G. Brik, E. Brown, and U. Hommerich, “Synthesis and optical properties of infrared emitting YF3:Nd nanoparticles,” J. Appl. Phys. 106(6), 063118 (2009). [CrossRef]

11.

Y. T. Lim, S. Kim, A. Nakayama, N. E. Stott, M. G. Bawendi, and J. V. Frangioni, “Selection of quantum dot wavelengths for biomedical assays and imaging,” Mol. Imaging 2(1), 50–64 (2003). [CrossRef] [PubMed]

12.

A. L. Rogach, A. Eychmüller, S. G. Hickey, and S. V. Kershaw, “Infrared-emitting colloidal nanocrystals: synthesis, assembly, spectroscopy, and applications,” Small 3(4), 536–557 (2007). [CrossRef] [PubMed]

13.

S. Tanabe, T. Kouda, and T. Hanada, “Energy transfer and 1.3 µm emission in Pr-Yb codoped tellurite glasses,” J. Non-Cryst. Solids 274(1-3), 55–61 (2000). [CrossRef]

14.

A. Kermaoui and F. Pelle, “Synthesis and infrared spectroscopic properties of Tm3+-doped phosphate glasses,” J. Alloy. Comp. 469(1-2), 601–608 (2009). [CrossRef]

15.

X. P. Chen, W. J. Zhang, and Q. Y. Zhang, “Towards efficient upconversion and downconversion of NaYF4: Ho3+, Yb3+ phosphors,” Physica B 406(6-7), 1248–1252 (2011). [CrossRef]

16.

X. Zou and H. Toratani, “Dynamics and mechanics of up-conversion processes in Yb3+ sensitized Tm3+- and Ho3+-doped fluorozircoaluminate glasses,” J. Non-Cryst. Solids 181(1-2), 87–99 (1995). [CrossRef]

17.

M. C. Tan, J. Connolly, and R. E. Riman, “Optical efficiency of short wave infrared emitting phosphors,” J. Phys. Chem. C 115(36), 17952–17957 (2011). [CrossRef]

18.

G. Wang, W. Qin, J. Zhang, L. Wang, G. Wei, P. Zhu, and R. Kim, “Controlled synthesis and luminescence properties from cubic to hexagonal NaYF4: Ln3+ (Ln=Eu and Yb/Tm) microcrystals,” J. Alloy. Comp. 475(1-2), 452–455 (2009). [CrossRef]

19.

J. C. Zhou, Z. L. Yang, W. Dong, R. J. Tang, L. D. Sun, and C. H. Yan, “Bioimaging and toxicity assessments of near-infrared upconversion luminescent NaYF4:Yb,Tm nanocrystals,” Biomaterials 32(34), 9059–9067 (2011). [CrossRef] [PubMed]

20.

D. J. Naczynski, M. C. Tan, M. Zevon, B. Wall, J. Kohl, A. Kulesa, S. Chen, C. M. Roth, R. E. Riman, and P. V. Moghe are preparing a manuscript to be called “Rare-earth doped biologic composites as shortwave infrared reporters in vivo”.

21.

B. D. Cullity and S. R. Stock, Elements of X-ray Diffraction (Prentice Hall, 2001).

22.

J. Solé, L. Bausa, and D. Jaque, An Introduction to the Optical Spectroscopy of Inorganic Solids (John Wiley, 2005).

23.

G. A. Hebbink, J. W. Stouwdam, D. N. Reinhoudt, and F. van Veggel, “Lanthanide(III)-doped nanoparticles that emit in the near-infrared,” Adv. Mater. 14(16), 1147–1150 (2002). [CrossRef]

24.

Y. S. Tver’yanovich, “Concentration quenching of luminescence of rare-earth ions in chalcogenide glasses,” Glass Phys. Chem. 29(2), 166–168 (2003). [CrossRef]

25.

X. P. Chen, X. Y. Huang, and Q. Y. Zhang, “Concentration-dependent near-infrared quantum cutting in NaYF4: Pr3+, Yb3+ phosphor,” J. Appl. Phys. 106(6), 063518 (2009). [CrossRef]

OCIS Codes
(160.0160) Materials : Materials
(160.4760) Materials : Optical properties
(160.5690) Materials : Rare-earth-doped materials

ToC Category:
Fluorescent and Luminescent Materials

History
Original Manuscript: January 31, 2013
Revised Manuscript: March 4, 2013
Manuscript Accepted: March 5, 2013
Published: April 3, 2013

Citation
Bryan van Saders, Lara Al-Baroudi, Mei Chee Tan, and Richard E. Riman, "Rare-earth doped particles with tunable infrared emissions for biomedical imaging," Opt. Mater. Express 3, 566-573 (2013)
http://www.opticsinfobase.org/ome/abstract.cfm?URI=ome-3-5-566


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References

  1. A. Jha, B. Richards, G. Jose, T. Teddy-Fernandez, P. Joshi, X. Jiang, and J. Lousteau, “Rare-earth ion doped TeO2 and GeO2 glasses as laser materials,” Prog. Mater. Sci.57(8), 1426–1491 (2012). [CrossRef]
  2. S. Tanabe, “Rare-earth-doped glasses for fiber amplifiers in broadband telecommunication,” C. R. Chim.5(12), 815–824 (2002). [CrossRef]
  3. B. S. Richards, “Luminescent layers for enhanced silicon solar cell performance: Down-conversion,” Sol. Energy Mater. Sol. Cells90(9), 1189–1207 (2006). [CrossRef]
  4. K. Deng, T. Gong, L. Hu, X. Wei, Y. Chen, and M. Yin, “Efficient near-infrared quantum cutting in NaYF4: Ho3+, Yb3+ for solar photovoltaics,” Opt. Express19(3), 1749–1754 (2011). [CrossRef] [PubMed]
  5. A. M. Smith, M. C. Mancini, and S. Nie, “Bioimaging: second window for in vivo imaging,” Nat. Nanotechnol.4(11), 710–711 (2009). [CrossRef] [PubMed]
  6. S. Heer, K. Kömpe, H. U. Güdel, and M. Haase, “Highly efficient multicolor upconversion emission in transparent colloids of lanthanide-doped NaYF4 nanocrystals,” Adv. Mater.16(23-24), 2102–2105 (2004). [CrossRef]
  7. R. Weissleder, “A clearer vision for in vivo imaging,” Nat. Biotechnol.19(4), 316–317 (2001). [CrossRef] [PubMed]
  8. J. V. Frangioni, “In vivo near-infrared fluorescence imaging,” Curr. Opin. Chem. Biol.7(5), 626–634 (2003). [CrossRef] [PubMed]
  9. D. J. Naczynski, T. Andelman, D. Pal, S. Chen, R. E. Riman, C. M. Roth, and P. V. Moghe, “Albumin nanoshell encapsulation of near-infrared-excitable rare-Earth nanoparticles enhances biocompatibility and enables targeted cell imaging,” Small6(15), 1631–1640 (2010). [CrossRef] [PubMed]
  10. M. C. Tan, G. A. Kumar, R. E. Riman, M. G. Brik, E. Brown, and U. Hommerich, “Synthesis and optical properties of infrared emitting YF3:Nd nanoparticles,” J. Appl. Phys.106(6), 063118 (2009). [CrossRef]
  11. Y. T. Lim, S. Kim, A. Nakayama, N. E. Stott, M. G. Bawendi, and J. V. Frangioni, “Selection of quantum dot wavelengths for biomedical assays and imaging,” Mol. Imaging2(1), 50–64 (2003). [CrossRef] [PubMed]
  12. A. L. Rogach, A. Eychmüller, S. G. Hickey, and S. V. Kershaw, “Infrared-emitting colloidal nanocrystals: synthesis, assembly, spectroscopy, and applications,” Small3(4), 536–557 (2007). [CrossRef] [PubMed]
  13. S. Tanabe, T. Kouda, and T. Hanada, “Energy transfer and 1.3 µm emission in Pr-Yb codoped tellurite glasses,” J. Non-Cryst. Solids274(1-3), 55–61 (2000). [CrossRef]
  14. A. Kermaoui and F. Pelle, “Synthesis and infrared spectroscopic properties of Tm3+-doped phosphate glasses,” J. Alloy. Comp.469(1-2), 601–608 (2009). [CrossRef]
  15. X. P. Chen, W. J. Zhang, and Q. Y. Zhang, “Towards efficient upconversion and downconversion of NaYF4: Ho3+, Yb3+ phosphors,” Physica B406(6-7), 1248–1252 (2011). [CrossRef]
  16. X. Zou and H. Toratani, “Dynamics and mechanics of up-conversion processes in Yb3+ sensitized Tm3+- and Ho3+-doped fluorozircoaluminate glasses,” J. Non-Cryst. Solids181(1-2), 87–99 (1995). [CrossRef]
  17. M. C. Tan, J. Connolly, and R. E. Riman, “Optical efficiency of short wave infrared emitting phosphors,” J. Phys. Chem. C115(36), 17952–17957 (2011). [CrossRef]
  18. G. Wang, W. Qin, J. Zhang, L. Wang, G. Wei, P. Zhu, and R. Kim, “Controlled synthesis and luminescence properties from cubic to hexagonal NaYF4: Ln3+ (Ln=Eu and Yb/Tm) microcrystals,” J. Alloy. Comp.475(1-2), 452–455 (2009). [CrossRef]
  19. J. C. Zhou, Z. L. Yang, W. Dong, R. J. Tang, L. D. Sun, and C. H. Yan, “Bioimaging and toxicity assessments of near-infrared upconversion luminescent NaYF4:Yb,Tm nanocrystals,” Biomaterials32(34), 9059–9067 (2011). [CrossRef] [PubMed]
  20. D. J. Naczynski, M. C. Tan, M. Zevon, B. Wall, J. Kohl, A. Kulesa, S. Chen, C. M. Roth, R. E. Riman, and P. V. Moghe are preparing a manuscript to be called “Rare-earth doped biologic composites as shortwave infrared reporters in vivo”.
  21. B. D. Cullity and S. R. Stock, Elements of X-ray Diffraction (Prentice Hall, 2001).
  22. J. Solé, L. Bausa, and D. Jaque, An Introduction to the Optical Spectroscopy of Inorganic Solids (John Wiley, 2005).
  23. G. A. Hebbink, J. W. Stouwdam, D. N. Reinhoudt, and F. van Veggel, “Lanthanide(III)-doped nanoparticles that emit in the near-infrared,” Adv. Mater.14(16), 1147–1150 (2002). [CrossRef]
  24. Y. S. Tver’yanovich, “Concentration quenching of luminescence of rare-earth ions in chalcogenide glasses,” Glass Phys. Chem.29(2), 166–168 (2003). [CrossRef]
  25. X. P. Chen, X. Y. Huang, and Q. Y. Zhang, “Concentration-dependent near-infrared quantum cutting in NaYF4: Pr3+, Yb3+ phosphor,” J. Appl. Phys.106(6), 063518 (2009). [CrossRef]

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