Up conversion luminescence of Yb3+–Er3+ codoped CeO2 nanocrystals with imaging applications

Journal of Luminescence 132 (2012) 743–749

Contents lists available at SciVerse ScienceDirect

Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin

Up conversion luminescence of Yb3 þ –Er3 þ codoped CeO2 nanocrystals with imaging applications Jung-Hyun Cho a, Michael Bass a,n, Suresh Babu b,1, Janet M. Dowding c, William T. Self c, Sudipta Seal b a

CREOL, The College of Optics and Photonics, University of Central Florida, Orlando, FL-32816, USA Advanced Materials Processing and Analysis Center, Department of Mechanical, Materials and Aerospace Engineering, University of Central Florida, Orlando, FL-32816, USA c Department of Molecular Biology and Microbiology, Burnett School of Biomedical Sciences, University of Central Florida, Orlando, FL-32816, USA b

a r t i c l e i n f o

abstract

Article history: Received 28 July 2011 Received in revised form 12 October 2011 Accepted 4 November 2011 Available online 12 November 2011

The effects of Yb3 þ doping on up conversion in Yb3 þ –Er3 þ co-doped cerium oxide nanocrystals are reported. Green emission around 545 and 560 nm attributed to the 2H11/2, 4S3/2-4I15/2 transitions and red emission around 660 and 680 nm due to 4F9/2- 4I15/2 transitions under 975 nm excitation were studied at room temperature. Both green and red emission intensities increase as the Yb3 þ concentration increases from 0%. Emission strength starts to decrease after the Yb3 þ concentration exceeds a critical amount. The green emission strength peaks around 1% Yb3 þ concentration while the red emission strength peaks around 4%. An explanation of competition between different decay mechanisms is presented to account for the luminescence dependence on Yb3 þ concentration. Also, the application of up converting nanoparticles in biomedical imaging is demonstrated. & 2011 Elsevier B.V. All rights reserved.

Keywords: Up conversion Nano particles Energy transfer effects Imaging

1. Introduction Cerium oxide nanoparticles have been shown to have therapeutic effects in cancer treatment—enhancing the effect of radiation on cancerous cells while reducing damage to normal cells [1,2]. A major problem in developing therapies in which nanoparticles are used is determining the mechanism whereby they interact with and potentially alter the biology of the cells. This requires knowing where the nanoparticles are located (are they near, on or within the cells), at what rate are they taken up by the cells and are they present singly or as agglomerates. Up conversion (UC) due to rare earth dopant ions in the nanoparticles can be detected while using near infrared light excitation making the presence and location of doped nanoparticles detectable using optical microscopy. Unlike down conversion, which typically requires UV (ultraviolet) or short wavelength visible light as an excitation source, low power infrared light sources can be used to generate visible light through up conversion. Consequently, up conversion imaging is less likely to have the shortcomings of traditional fluorescence imaging such as background fluorescence, photo-bleaching and photo-damage. In addition, low power infrared wavelengths are less damaging to biological systems. n

Corresponding author. Tel.: þ1 407 718 2843. E-mail addresses: [email protected] (M. Bass), [email protected] (S. Seal). 1 Present Address: Center for Nanoscience and Technology, Madanjeet School of Green Energy Technologies, Pondicherry University, Puducherry 605 014, India. 0022-2313/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2011.11.011

UC in rare-earth (RE) doped materials was first reported more than fifty years ago [3]. Soon after, the mechanism for UC was made one or two orders of magnitude more efficient using ytterbium (Yb3 þ ) as a sensitizer ion. The Yb3 þ ions would absorb near infrared light and transfer the energy absorbed to an emitter ion such as erbium (Er3 þ ), holmium (Ho3 þ ) or thulium (Tm3 þ ) [4–9]. The studies in Refs. [4–9] involved crystalline and glass hosts. UC in a co-doped niobium based glass host demonstrated energy transfer assisted up conversion with high efficiency indicating a dependence on the glass host [10]. Energy transfer UC in co-doped chalcogenide glass was also reported [11]. In Refs. [10,11] the excitation source was an Nd:YAG laser operating at 1.064 mm, which means that the Yb3 þ donor ions were excited where their absorption was quite weak. However, a clear nonlinear dependence of Er3 þ emission in the green and red was observed indicative of the energy transfer process in the glass hosts. In addition, in Ref. [11] where the host was a chalcogenide glass the 1.064 mm pumping enabled observation of up conversion at new wavelengths, 830 and 925 nm. With the advent of nanocrystal science and identification of their potential applications studies of up UC in nanocrystals have been reported [12–14]. Ref. [12] discusses several applications of UC and describes up conversion in suspensions of nanoparticles. Systems were also studied in which rare earth doped nanoparticles are suspended in transparent glass ceramics [13,14]. Ref. [13] used 980 nm light to directly excite the Er3 þ dopant ions in b-PbF2:Er3 þ nanocrystals suspended in a PbGeO3-PbF2-CdF2 transparent glass ceramic. Several visible UC emissions were seen

744

J.-H. Cho et al. / Journal of Luminescence 132 (2012) 743–749

from the blue to the near IR parts of the spectrum. In Ref. [14] b-PbF2:Ho3 þ was studied in the same transparent glass ceramic host. In Refs. [13,14] the absorbing ion is also the emitting ion and the benefit of co-doping with Yb3 þ ions was not explored as it is in this paper. Nevertheless good UC was detected and mechanisms proposed. A detailed examination of UC is reported in this paper for ceria (CeO2) nanoparticles doped with Yb3 þ donor (sensitizer) and Er3 þ acceptor (visible light emitting) ions. The effect of Yb3 þ donor ion density is presented as this is a critical factor in determining the efficiency of up conversion [15,16]. Co-doping with a donor ion of Yb3 þ enables UC of 975 nm pump light because Yb3 þ absorbs this wavelength strongly and can efficiently transfer the absorbed energy to Er3 þ emitter ions. Evidence is presented of quenching and other non-radiative decay mechanisms involved in the fluorescence dynamics of Yb3 þ , Er3 þ :CeO2 and a model proposed. We demonstrate the effects of Yb3 þ doping density on up conversion in Er3 þ /Yb3 þ co-doped cerium oxide nanocrystals and shows the potential of up conversion in such particles to make their presence and location easily detectable.

observed indicating the formation of single phased cerium oxide. Using the Scherrer formula the mean nanoparticle size was found to be 35 nm. To understand size modifications, high resolution transmission electron microscopic (HRTEM) studies were carried out. The mean size of particles was found to be 30 nm (see Fig. 2). The lattice planes seen in that figure correspond to those of (111) planes. 2.2. Spectroscopic measurements The absorption spectrum of 20%Yb, 2%Er doped CeO2 nanoparticles was obtained using a Cary 500 spectrophotometer. Nanoparticle powder was dispersed on a strip of transparent double-sided tape placed on a glass microscope slide and placed in the spectrophotometer for measurement.

2. Experimental 2.1. Nanocrystal preparation Doped metal oxides were prepared by a chemical precipitation technique. Metal nitrates of cerium, ytterbium and erbium were used as the precursor. To 0.285 M cerium (III) nitrate in 25 ml of aqueous solution, erbium (III) nitrate solution was added (Ce:Er molar ratio 1:0.02) and subsequently varying concentrations of ytterbium (III) nitrate was added. The Ce:Yb molar ratios were 0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.1, 0.15 and 0.20. To the above solution 1 N ammonia solution was added to maintain the pH value of 10. After about 4 h of stirring the solution was dried over a hot plate and the dried powder was annealed at 900 1C for 2 h. X-ray diffraction studies were carried out using Rigaku D-Max B diffractometer to understand the modification in crystal structure. The crystal structure of 15%Yb, 2%Er:CeO2 annealed at 900 1C for 2 h is shown in the X-ray diffraction data in Fig. 1. The fluorite structure with the crystalline planes of (111), (200), (220), (311), (222), (400), (331) and (420) matches with that of CeO2 (JCPDS # 34-0394). No separate phase formation for Er and Yb oxide was

Fig. 2. HRTEM images of CeO2:2%Er:15%Yb.

14 12

Absorption (a.u.)

(222)

(311)

(220)

(200)

Intensity (a.u.)

(111)

10 8 6 4 2

(331) (420)

(400)

0 -2 920 20

30

40

50

60

70

80

2θ Fig. 1. X-ray diffraction pattern of CeO2:2%Er:15%Yb exhibiting fluorite structure.

940

960

980

1000

1020

Wavelength (nm) Fig. 3. Measured absorption spectrum of 2%Er, 20%Yb:CeO2. The main absorption peak is at 975 nm due to the 2F7/2-2F5/2 transition of Yb3 þ . The vertical scale is in arbitrary units.

J.-H. Cho et al. / Journal of Luminescence 132 (2012) 743–749

A Compact Array Spectrophotometer (Instrument Systems, CAS-140B) and integrating sphere combination was used to measure the emission spectra. Powders of nanoparticles were packed in a transparent acrylic sample holder, carefully mixed with refractive index matching oil to minimize backscattered light, and then placed in the integrating sphere. A 975 nm nearinfrared emitting diode laser was used to excite the samples. Upconversion luminescence was collected by the integrating sphere and then sent to the spectrophotometer. A Spectra Physics PRO-290 Nd:YAG laser and MOPO-730 were used to generate pulses at 975 nm of approximately 4 ns duration with 10 Hz pulse repetition frequency for the decay lifetime measurements. The 975 nm light was focused at the samples and the emitted light was collected by a fast detector (Hamamatsu

745

PMT H6780-20) which was attached to an Acton SpctraPro-300i Monochromator. The signal was then recorded with a Tektronix 2440 oscilloscope.

2.3. Cell culture A549 lung carcinoma cells were obtained from American Type Culture Collection (Manassa, VA, USA). These cells were cultured in Delbecco’s modification of Eagle’s medium (DMEM) (Mediatech, Inc, Manassa, VA, USA) supplemented with 10% fetal bovine serum (FBS) (Equi-tech Bio, Kerrville, TX, USA) and 100 IU mL  1 penicillin (Mediatech, Inc, Manassa, VA, USA). The cell cultures were maintained at 37 1C in a humidified incubator under a 5% atm of CO2.

2.5

20

NR NR

15

5% 4% 3%

0.5

2%

10

ET ET

5

1%

I9/2

4I 11/2

4

I13/2

0%

0.0 500

NR

GSA

1.0

2F 5/2

F9/2

4

ET

678 nm

6%

547 nm

7%

526 nm

Energy (103 cm-1)

Intensity (a.u.)

10% 1.5

4

ESA

15% Yb3+

ESA

NR

2.0

4F 7/2 2H 11/2 4S 3/2

550

600

650

0

700

2

F7/2

Wavelength (nm) Fig. 4. Emission spectra of 2%Er:CeO2 doped with different Yb3 þ concentrations. Green emission is centered around 545 and 560 nm and red emission is centered around 660 and 680 nm.

Yb3+

Er3+

4I 15/2

Fig. 6. Schematic energy level diagram of the Yb3 þ /Er3 þ codoped system diagraming of the main processes that are responsible for the green and red upconversion emission.

0.3

547 nm 0.2

Intensity (a.u.)

0.1

0.0 0

2

4

6

8

10

12

14

16

678 nm

1.5 1.0 0.5 0.0 0

2

4

6

8

10

12

14

16

Yb concentration (mol%) Fig. 5. Strength of the green emission at 547 nm (top) and red emission at 678 nm (bottom) from 2%Er doped ceria as a function of Yb3 þ dopant density. The green emission intensity peaks around 1% and the red emission intensity peaks around 4% Yb3 þ .

746

J.-H. Cho et al. / Journal of Luminescence 132 (2012) 743–749

2.4. Microscopy

3. Theory and discussion

Samples for microscopy were prepared as follows: Cells were grown to approximately 60–70% confluence. A sterile H2O suspension of 10 mM doped-CeO2 nanoparticles was sonicated for 60 min before adding to cultures. After 24 h treatment with this solution diluted to 500 mM CeO2 nanoparticle concentration, cells were washed 2 times in Dulbeccos’s Phosphate Buffered Saline, (Mediatech, Inc, Manassa, VA, USA), trypsinized and transferred to glass coverslips. After a further 24 h growth, cells were washed 2 times in saline, fixed in cold (20 1C) methanol for 10 min at RT then washed twice with sterile water and allowed to dry.

Many studies have shown that in most ytterbium co-doped systems near-infrared light around 980 nm can be absorbed by the sensitizer ion Yb3 þ and then transferred to other active ions leading to efficient up conversion [4–9,15–19]. As shown in Fig. 3 the absorption spectrum of 20% Yb, 2%Er:CeO2 suggests that approximately 975 nm is the optimum wavelength for pumping. The 2F7/2-2F5/2 transition of Yb3 þ is responsible for this absorption [15]. The visible emission spectra of 2%Er:CeO2 co-doped with different Yb3 þ concentrations when pumped with 975 nm light are shown in Fig. 4. Green emission is centered at 545 and 560 nm due to the 2H11/2, 4S3/2-4I15/2 transitions. Red emission from the 4F9/2-4I15/2 transition is also evident centered at 660 and 680 nm. Fig. 5 shows the Yb3 þ concentration dependence of up conversion luminescence strength when the concentration of Er3 þ was fixed at 2% and the Yb3 þ concentration was varied from 0 to 15%. The visible emission intensities of both red and green light increased as the Yb3 þ concentration was increased from 0% but the emission strength decreased after the Yb3 þ concentration exceeded a critical amount. The green emission intensity peaked around 1% Yb3 þ concentration while the red emission intensity peaked around 4% (See Fig. 5). An energy level diagram is sketched in Fig. 6 for the Yb3 þ /Er3 þ co-doped system showing the up conversion processes that are primarily responsible for the green and red emission when using 975 nm excitation. The excitation processes can be explained in several steps. First, the 975 nm photons are absorbed by Yb3 þ ions (2F7/2-2F5/2). A large part of this energy is then efficiently transferred to the Er3 þ ions due to the large spectral overlap between Yb3 þ emission and the Er3 þ absorption. This enables the 4 I15/2-4I11/2 transition of the Er3 þ ions. Another energy transfer (ET) from Yb3 þ to Er3 þ causes the 4I11/2-4F7/2 transition. Next, non-radiative decay populates the green emitting levels 2H11/2, 4 S3/2. Finally, the 2H11/2, 4S3/2-4I15/2 transitions generate green emissions. For the red emission, there are two different pathways for populating the 4F9/2 level. First, similar to the green emission channel, the ETs from the Yb3 þ ions populate 4F7/2 level of Er3 þ

35 30

Red/Green

25 20 15 10 5 0 0

2

4

6

8

10

12

14

16

Yb concentraion (mol %) Fig. 7. Concentration dependent red to green emission ratio of 2%Er3 þ and Yb3 þ co doped CeO2 as a function of Yb3 þ concentration. As Yb3 þ doping increases the red to green luminescence ratio increases linearly.

547 nm

400

Decay time constant (µs)

300 200 100 0 0

2

4

6

8

10

12

400

14

16

678 nm

300 200 100 0 0

2

4

6

8

10

12

14

16

Yb concentration (mol%) Fig. 8. Fluorescence lifetime of 2%Er3 þ doped ceria as a function of Yb3 þ dopant density. Solid lines are a guide to the eyes. The fluorescent lifetime of 2%Er3 þ doped material is longest for Yb3 þ dopant density near 1% and decreases as the Yb3 þ concentration increases.

J.-H. Cho et al. / Journal of Luminescence 132 (2012) 743–749

n_ 1Yb ¼

n2Yb

sabs Yb

tYb

Ipump ðtÞ ðn n Þ þ g1 n1Er n2Yb þ g2 n2Er n2Yb hnpump 1Yb 2Yb

n_ 2Yb ¼ n_ 1Yb n_ 1Er ¼

n2Er

t2

n_ 2Er ¼ 

t2

Intensity

0.5

0.0

n3Er

t3

0.6

0.8

1.0

1.2

1.4

1.6

1.0

0.5

ð3Þ

þ A32 n3Er þ g1 n1Er n2Yb g2 n2Er n2Yb

ð4Þ

þ g2 n2Er n2Yb Cn23Er Dn3Er n2Yb

ð5Þ

where n1Yb, n2Yb, n1Er, n2Er, and n3Er are the populations of the 2F7/2 and 2F5/2 levels of Yb3 þ and of the 4I15/2 ground level, 4I11/2 and 4S3/2 3þ of Er3 þ , respectively; sabs Yb is the absorption cross section for Yb and ti are the corresponding decay lifetimes, Aji are the decay rates from j to i; g1 and g2 are the energy transfer rates from Yb3 þ to Er3 þ ; C is the concentration dependent non-radiative decay coefficient, and D represent the concentration dependent cross-relaxation coefficient. These simplified rate equations ignored several other energy levels and back-transfer. Cross-relaxations were not explicitly included. However terms proportional to Cn23Er and Dn3Ern2Yb were added to examine the effect of concentration dependent quenching and non-radiative decay on the fluorescence dynamics. Eqs. (1)–(5) were solved numerically using Mathematica and applied to fit the experimental data with the parameters shown in Fig. 9. Good agreement with the data is obtained only when the Cn23Er and Dn3Ern2Yb terms were included. This implies that the quenching and other non-radiative decay mechanisms are involved in the fluorescence dynamics of Yb3 þ ,Er3 þ :CeO2. The input power dependence of the luminescence strength provides better understanding of the energy transfer mechanism. The visible luminescence intensity is proportional to the nth power of the pump intensity as, Ivis pInIR

0.4

Time (ms)

0.0 0.0

n_ 3Er ¼ 

0.2

ð2Þ

þA31 n3Er g1 n1Er n2Yb þCn23Er þ Dn3Er n2Yb

n2Er

ð1Þ

1.0

Intensity

ions. Non-radiative decay then populates the red emitting level (4F9/2). An alternate pathway to populate the 4F7/2 is as follows: ET from Yb3 þ to Er3 þ makes the 4I15/2-4I11/2 transition. Then the transition 4I11/2-4I13/2 takes place due to the nonradiative decay. The 4I13/2-4F9/2 transition occurs by ET from Yb3 þ (2F5/2-2F7/2) and then the 4F9/2-4I15/2 transition gives red emission. It is obvious that as Yb3 þ doping is introduced, both red and green emission increase due to the efficient absorption of 975 nm photons by the Yb3 þ ions and the resonant ET to Er3 þ ions. However, when we increase the concentrations of Yb3 þ up to 4%, the green emission starts to decrease while the red emission continues increasing. This can be explained by an increase in non-radiative decay from the common intermediate state of Er3 þ (4I11/2) to the 4I13/2 level resulting in enhancement in the red emission and the reduction of the green emission [15]. A concentration dependent process is clearly shown in Fig. 7 However, excessive Yb3 þ doping can cause the entire luminescence to decrease due to the aggregation of Yb3 þ ions and reduced distance between Yb3 þ ions. Yb3 þ clusters can act as trapping centers causing dissipation of absorbed energy non-radiatively rather than transferring it to the Er3 þ ions. Additionally, fluorescence quenching occurs through cooperative luminescence between Yb3 þ –Yb3 þ pairs [17,18]. The fluorescence lifetime was measured for all the samples. As shown in Fig. 8 the fluorescent lifetime of 2% Er3 þ doped material is longest for Yb3 þ dopant density near 1% and decreases as the Yb3 þ concentration increases. The dynamics of the fluorescence decay can be explained by simplified rate equations as shown in Eqs. (1)–(5).

747

0.2

0.4

0.6

Time (ms) Fig. 9. Normalized emission decay of Er3 þ and Yb3 þ co-doped ceria nanoparticles excited at 975 nm with a 4-ns pulse. The emission at 547 nm (circle) and simulation data (solid line) are shown for the samples with different Yb3 þ doping concentrations: (a) 2%Er, 2%Yb:CeO2, (b) 2%Er, 15%Yb:CeO2. The simulation data were acquired by numerically solving the rate Eqs. (1)–(5). The fitting parameters are (a) g1 ¼8  10  18, g2 ¼ 1  10  17, C ¼ 5  10  3, D¼ 2  10  6 and (b) g1 ¼5  10  16, g2 ¼ 1  10  17, C ¼ 6  10  3, D¼ 2  10  9.

where n is the number of absorbed pump photons. The value of n should be close to 2 for two photon processes such as considered here. However, the values of n shown in Fig. 10a and b are between 1 and 2. For the green emission, the n values are 1.79 (0%Yb), 1.58 (1%Yb), 1.49 (3%Yb), 1.49 (5%Yb), 1.49 (7%Yb), 1.52 (10%Yb) and 1.65 (15%Yb) and for the red emission, the values are 1.76 (0%Yb), 1.6 (1%Yb), 1.49 (3%Yb), 1.5 (5%Yb), 1.5 (7%Yb), 1.53 (10%Yb) and 1.63 (15%Yb). This data can be understood by considering the competition between linear decay and the up conversion processes for the population of the intermediate state 4 I11/2 of Er3 þ ions [19]. The up conversion mechanism for the Yb3 þ /Er3 þ co-doped system regarding the competition between linear decay and up conversion was examined in detail in Ref. [20]. Because of the competition between linear decay and up conversion, the luminescence intensity does not exactly follow the nth power rule. Based on the simplified three-level system described in Ref. [20], when linear decay is dominant, Ivis is

748

J.-H. Cho et al. / Journal of Luminescence 132 (2012) 743–749

e4 Green e3

ln(peak intensity (a.u.))

e2 e1 e0 e-1 0% Yb n=1.79 1% Yb n=1.58 3% Yb n=1.49 5% Yb n=1.49 7% Yb n=1.49 10% Yb n=1.52 15% Yb n=1.65

e-2 e-3 e-4 e-5 e1

e2 e3 e4 ln(Average pump power (mW))

e5

Red e5

ln(peak intensity (a.u.))

e4 e3 e2 e1

0% n=1.76 1% n=1.6 3% n=1.49 5% n=1.5 7% n=1.5 10% n=1.53 15% n=1.63

e0 e-1 e-2 e1

e2 e3 e4 ln(Average pump power (mW))

e5

Fig. 10. Logarithmic dependence of peak intensity of the (a) green (547 nm) and (b) red (678 nm) upconversion luminescence as a function of input power for the 2%Er:CeO2 with different Yb3 þ concentrations.

proportional to I2ir. If up conversion is dominant Ivis will be proportional to Iir. That means, when the two photon up conversion process becomes dominant the n value (slopes of the lines in Fig. 10a and b) will become closer to 1. When up conversion is not efficient or other processes are assisting linear decay, the n value will be closer to 2. For both green and red up conversion emission of the different Yb3þ co-doped 2%Er:CeO2 samples n values first decrease and then increase again as the Yb3 þ concentration increases. When Yb3þ is first introduced to enhance the two-photon up conversion process, the n value starts to decrease because up conversion is dominant. However, when more Yb3 þ ions are added, processes such as quenching start to take over and increase linear decay of the intermediate energy level in the Er3 þ ions. Additionally in the high power region the slopes in Fig. 10a and b become smaller for all cases. Suyver et. al. [21] showed that in the high-power limit, Ivis is proportional to Iir. This is exactly the case observed for the different Yb co-doped 2%Er:CeO2. At high inputs all have n values close to 1 regardless of the Yb3 þ doping concentration.

Fig. 11. Optical microscopy of lung cancer cells (A549) treated with doped ceria nanoparticles exposed with both white light from the microscope lamp and 975 nm light from a diode laser. Nanoparticle presence is made clear by the particles that emit visible light and are most often found around the outside of the cells. Cells were exposed to nanoparticles for 24 h, subsequently subcultured to remove nanoparticles not taken up by the cells and replated as described in the section on methods.

To demonstrate the potential of this technique for easy detection of the presence and locations of nanoparticles, the microscope set up described in Ref. [22] was used. It involved adding to a conventional optical microscope’s illumination light from the 975 nm near-infrared diode laser. Images such as shown in Fig. 11 were obtained where the nanoparticles emitted green light by up conversion. The most important parts of such imagery are the weakest emitters seen as they may be due to single nanoparticles. The very bright emitters represent clusters of nanoparticles. Though agglomeration of nanoparticles in vivo is commonly observed, for biomedical applications such clustering will have to be improved. Additionally, using an excitation source in the infrared is beneficial for biological imaging. Infrared allows for enhanced penetration into biological tissues with decreased damage to tissues due to lower photon energy.

4. Results and conclusions The concentration dependent up conversion luminescence of nano cystalline Yb3 þ , Er3 þ :CeO2 and its application were studied.

J.-H. Cho et al. / Journal of Luminescence 132 (2012) 743–749

As Yb3 þ doping is introduced both red (4F9/2-4I15/2) and green (2H11/2, 4S3/2-4I15/2) up conversion emission increase due to efficient absorption of near-infrared photons by the Yb3 þ ions and resonant ET to Er3 þ ions. However, as we increase the Yb3 þ doping up to 4%, the non-radiative decay from the common intermediate state of Er3 þ (4I11/2) to the 4I13/2 level increases causing enhancement in the red emission and the reduction of the green emission. Further Yb3 þ doping results in reducing the entire luminescence emitted due to fluorescence quenching and non-radiative decay resulting from Yb3 þ ion clusters. The potential for using up conversion to locate nanoparticles in biologic systems was demonstrated.

Acknowledgments Partial support came from NSF NIRT: CBET0708172 and 0930170. References [1] [2] [3] [4]

C. Korsvik, S. Patil, S. Seal, W. Self, Chem. Comm. 10 (2007) 1056. R.W. Tarnuzzer, J. Colon, S. Patil, S. Seal, Nano Lett. 5 (12) (2005) 2573. N. Bloembergen, Phys. Rev. Lett. 2 (3) (1959) 84. F. Auzel, Annales des Telecommunications 24 (9–10) (1969) 363.

749

[5] J.E. Geusic, F.W. Ostermayer, H.M. Marcos, L.G. Van Uitert, J.P. van der Ziel, J. App. Phys. 1971 (42) (1958) 5. [6] H.J. Guggenheim, L.F. Johnson, App. Phys. Lett. 15 (2) (1969) 51. [7] L.F. Johnson, J.E. Geusic, H.J. Guggenheim, T. Kushida, S. Singh, L.G. Van Uitert, App. Phys. Lett. 15 (2) (1969) 48. [8] L.G. Van Uitert, S. Singh, H.J. Levinstein, L.F. Johnson, W.H. Grodkiewicz, J.E. Geusic, App. Phys. Lett. 15 (2) (1969) 53. [9] L. Esterowitz, A. Schnitzler, J. Noonan, J. Bahler, App. Opt. 7 (10) (1968) 2053. [10] A.S. Oliveria, M.T. de Araujo, A.S. Gouveia-Neto, A.S.B. Sombra, J.A. Medeiros Neto, N. Aranha, J. Appl. Phys. 83 (1) (1998) 604. [11] A.S. Oliveria, M.T. de Araujo, A.S. Gouveia-Neto, J.A. Medeiros Neto, A.S.B. Sombra, Y. Messaddeq, Appl. Phys. Lett. 72 (7) (1998) 753. ¨ [12] J.F. Suyver, A. Aebischer, D. Biner, P. Gerner, J. Grimm, S. Heer, K.W. Kramer, ¨ C. Reinhard, H.U. Gudel, Opt. Mater. 27 (2005) 1111. [13] A.S. Gouveia-Neto, E.B. da Costa, L.A. Bueno, S.J.L. Ribeiro, J. Alloys Comp. 375 (2004) 224. [14] A.S. Gouveia-Neto, E.B. da Costa, L.A. Bueno, S.J.L. Ribeiro, J. Lumin. 110 (2004) 79. [15] F. Vetrone, J.–C. Boyer, J.A. Capobianco, J. App. Phys 96 (1) (2004) 661. [16] H. Liu, L. Wang, S. Chen, Mat. Lett. 61 (17) (2007) 3629. [17] T. Hirai, T. Orikoshi, I. Komasawa, Chem. Mater. 14 (8) (2002) 3576. [18] L. Su, J. Xu, H. Li, L. Wen, Y. Wen, Y. Zhu, Z. Zhao, Y. Dong, G. Zhou, J. Si, Chem. Phys. Lett. 406 (1) (2005) 254. ¨ ¨ [19] M. Pollnau, D.R. Gamelin, S.R. Luthi, H.U. Gudel, Phys. Rev. B 61 (5) (2000) 3337. [20] Y. Lei, H. Song, L. Yang, L. Yu, Z. Liu, G. Pan, X. Bai, L. Fan, J. Chem. Phys. 123 (17) (2005) 174710. ¨ [21] J.F. Suyver, A. Aebischer, S. Garcı´a-Revilla, P. Gerner, H.U. Gudel, Phys. Rev. B 71 (2005) 125123/1-9. 22 S. Babu, J.-H. Cho, J.M. Dowding, E. Heckert, C. Komanski, S. Das, J. Colon, C.H. Baker, M. Bass, W.T. Self, S. Seal, Chem. Comm. 46 (37) (2010) 6915.