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Comparative Study of Upconverting Nanoparticles with Various Crystal Structures, Core/Shell Structures, and Surface Characteristics Yong Il Park, Sang Hwan Nam, Jeong Hyun Kim, Yun Mi Bae, Byeongjun Yoo, Hyung Min Kim, Ki-Seok Jeon, Joon Sig Choi, Kang Taek Lee, Yung Doug Suh, and Taeghwan Hyeon J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp3105248 • Publication Date (Web): 09 Jan 2013 Downloaded from http://pubs.acs.org on January 16, 2013

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The Journal of Physical Chemistry

Comparative Study of Upconverting Nanoparticles with Various Crystal Structures, Core/Shell Structures, and Surface Characteristics Yong Il Park,†,ǁ Sang Hwan Nam,‡,ǁ Jeong Hyun Kim,†,ǁ Yun Mi Bae,‡,§ Byeongjun Yoo,† Hyung Min Kim,‡, # Ki-Seok Jeon,‡ Joon Sig Choi,§ Kang Taek Lee,‡,* Yung Doug Suh,‡,* and Taeghwan Hyeon†,* †

Center for Nanoparticle Research, Institute for Basic Science, and School of Chemical and

Biological Engineering, Seoul National University, Seoul 151-742, Korea ‡

Laboratory for Advanced Molecular Probing (LAMP), Research Center for Convergence

Nanotechnology, Korea Research Institute of Chemical Technology, Daejeon 305-600, Korea §

ǁ

Department of Biochemistry, Chungnam National University, Daejeon 305-764, Korea

These authors contributed equally to this work.

#

Current address: Department of Bio & Nano Chemistry, Kookmin University, Seoul 136-702,

Korea *Address correspondence to [email protected]; [email protected]; [email protected].

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Abstract Upconverting nanoparticles (UCNPs) have been studied as novel bio-imaging probes owing to the absence of autofluorescence and excellent photostability. For practical applications, biocompatible UCNPs with high upconversion efficiency, bright luminescence, and good colloidal stability are desirable. Herein, we report quantitative and systematic study on the upconversion luminescence from a set of NaYF4:Yb3+,Er3+ based nanoparticles by varying crystal structures, core/shell structures, and surface ligands. Upconversion luminescent properties in colloidal solution and at the single-particle level were examined, and hexagonal phase core/shell UCNPs exhibited the most intense luminescence among various structures while the excellent photostability was observed in all different types of UCNPs. To optimize biomedical imaging capability of UCNPs, various surface coating strategies were tested. By quantitative spectroscopic measurements of surface-modified UCNPs in water, it was suggested that encapsulation with PEG-phospholipid was found to be effective in retaining both upconversion luminescence intensity and dispersibility in aqueous environment. Finally, UCNPs with different crystal structures were applied and compared in live cells.

Keywords imaging agents · lanthanides · luminescence · nanoparticles · nonlinear optics · upconversion


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Introduction Recently upconverting nanoparticles (UCNPs) have attracted much attention as novel luminescent nanomaterials. UCNPs emit visible photons by absorbing two or more near-infrared (NIR) photons.1−5 Thanks to such a unique luminescence mechanism, optical imaging with UCNPs has advantages over conventional fluorescence imaging with organic dyes or semiconductor quantum dots (QDs). The NIR excitation increases penetration depth and avoids autofluorescence of biological samples.6 Moreover, UCNPs show superior photostability exhibiting neither photoblinking nor photobleaching,7,8 and are less toxic than heavy metalcontaining QDs.9 In cellular imaging, the absence of autofluorescence and excellent photostability enhance signal to noise ratio, enabling single-particle level imaging and long-term tracking.7−15 Several reports on in vivo imaging using UCNPs have also been published.15−18 Recently, multimodal imaging probes based on UCNPs have been developed for more accurate imaging and diagnosis.7,19−24 For extensive applications to in vivo imaging probes, however, more optimal conditions have to be developed to enhance luminescence intensity. It is well known that hexagonal phase NaYF4 is the most efficient host matrix for upconversion.2−5 In bulk, upconversion efficiency of hexagonal phase NaYF4:Yb3+,Er3+ is about an order of magnitude higher than that of cubic phase counterpart.25 Researchers have reported on the synthesis of uniformly sized cubic phase UCNPs with NaYF4 host matrix,26 but the synthesis of pure hexagonal UCNPs of small sizes is still very challenging. Very recently, synthetic methods to obtain small and monodisperse hexagonal phase UCNPs have been developed.27−30 Addition of NaYF4 shell to NaYF4:Yb3+,Er3+ core also improved upconversion luminescence intensity.31−33 The UCNPs of core/shell structure showed luminescence enhancement because the shell can protect dopant ions and inhibit defect sites on the particle 3 ACS Paragon Plus Environment


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surface from luminescence quenching.34 Surface ligands also influence the upconversion luminescence in water.35 Generally, luminescence from UCNPs tends to decrease in aqueous environment. Therefore, for bio-imaging purpose, selection of proper stabilizing ligand is critical for preserving the brightness of UCNPs in aqueous media. Here we report on the comparative analysis of upconversion luminescence of uniformly sized UCNPs with varying crystal structures and core/shell structures. Optical properties of the UCNPs were measured and compared in colloidal solution and at the single-particle level. Furthermore, we compared the luminescence efficiency of water-dispersible UCNPs coated with various coating materials. On the basis of these results, we found the optimal combination of UCNPs and surface modification with high luminescence intensity for bio-imaging applications.

Experimental Section The experimental details are described in the Supporting Information. Briefly, the procedure for the synthesis of UCNPs, namely, cubic and hexagonal phase NaYF4:Yb3+,Er3+ and corresponding core/shell NaYF4:Yb3+,Er3+/NaGdF4 was adopted from the previous reports with some modifications.7,27 In order to make UCNPs water-dispersible, they were coated by PEGphospholipids,7 PO-PEG,36 silica,37 and mesoporous silica.38 The photoluminescence spectra of such various UCNP structures were measured by a home-made spectrometer equipped with a 980-nm laser (SDL-980LM-500T, Shanghai Dream Lasers Technology), a monochromator (HoloSpec f/1.8i, Kaiser Optical Systems), and a charge coupled device (CCD) camera (PIXIS 400BR, Princeton Instruments). For quantitative and systematic study on upconversion luminescence from UCNP solutions in hexane or water, ion concentrations of Y3+, Gd3+, Yb3+, 4 ACS Paragon Plus Environment

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and Er3+ were measured using inductively coupled plasma atomic emission spectroscopy (ICPAES). For imaging UCNPs, an epi-fluorescence microscope setup composed of an inverted microscope (TE2000-U, Nikon), an NIR (980 nm) diode laser (P161-600-980A, EM4), and an electron multiplying charge coupled device (EMCCD) camera (DV897DCS-BV, iXon, Andor Technology) was employed. The wide-field imaging of UCNPs internalized in HeLa cells were conducted using the same microscope setup equipped with an on-stage incubation chamber (Live Cell Instruments) that provides optimal conditions for live-cell imaging (5% CO2 and 37°C). Finally, in vitro cytotoxicity was assessed with MTT cell proliferation assay.

Results and Discussion NaYF4 nanoparticles have two kinds of crystal structures, which are cubic (α) and hexagonal (β) phases (Figure S1 in Supporting Information). Cubic phase NaYF4:Yb3+,Er3+ UCNPs were synthesized through thermal decomposition of metal-trifluoroacetates injected by a syringe pump at high temperature (310 oC).7,26 By using this method, uniformly sized cubic phase UCNPs could be synthesized. Recently, the we reported on the synthesis and application of cubic phase NaGdF4:Yb3+,Er3+/NaGdF4 core/shell UCNPs using a similar method.7 However, it was difficult to synthesize hexagonal phase UCNPs using the same scheme. It required reaction temperatures as high as boiling point of solvents,29,31,39 resulting in large sized UCNPs with mixed crystal phases (Figure S2 in Supporting Information). In this work, hexagonal phase NaYF4:Yb3+,Er3+ UCNPs were synthesized through thermal decomposition of metal-oleates by the “heat-up process,”27,40 which is very reproducible, environmentally friendly, and can be readily adapted for the large-scale production. The 30-nm sized uniform cubic and hexagonal phase 5 ACS Paragon Plus Environment


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NaYF4:Yb3+,Er3+ UCNPs were successfully synthesized (Figure 1a and 1b). The high-resolution transmission electron microscopy (HRTEM) images revealed the highly crystalline nature of these nanoparticles with d-spacing values of 0.315 nm for cubic phase and 0.515 nm for hexagonal phase, which match very well d111 value (0.316 nm) for cubic phase and d100 (0.515 nm) for hexagonal phase, respectively (Lower insets in Figure 1a and 1b). The electron diffraction (ED) patterns also confirmed the crystal structures (Upper inserts in Figure 1a and 1b). The X-ray diffraction (XRD) patterns shown in Figure 1c and 1d confirmed that the UCNPs are cubic phase (JCPDS 77-2042) and hexagonal phase (JCPDS 16-0334), respectively. Using the Sherrer formula, the sizes of UCNPs were estimated to be 27.5 nm and 28.6 nm for cubic and hexagonal phase, respectively, which are close to those determined by TEM. In order to examine the effect of shell on the upconversion luminescence, NaGdF4 shell was fabricated on the NaYF4:Yb3+,Er3+ core nanoparticles (Figure S3 in Supporting Information). In general, the addition of shell increases and stabilizes luminescence by protecting the surface exposed dopant ions and removing the defect sites.31−34 Dopant ions exposed on the particle surface tend to be quenched rapidly by solvents and surface ligands with high vibrational energy.35 Defect sites and impurities also cause luminescence quenching.34 We chose NaGdF4 as the shell matrix to introduce multimodality for future in vivo imaging applications. We have shown that Gd3+ ions in the matrix can enhance contrast in T1-weighted magnetic resonance imaging (MRI).7,19−21 In Figure 2, the photoluminescence spectra of UCNPs with different crystal structures are shown. Four types of UCNP samples, namely, cubic core, cubic core/shell, hexagonal core, and hexagonal core/shell UCNPs were dispersed in hexane and the spectra were obtained with 980nm excitation power of ~75 W/cm2. Every UCNP sample displayed sharp green and red emission 6 ACS Paragon Plus Environment

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bands as determined by the emitting levels of Er3+ ion (Figure S4 in Supporting Information), but with different relative intensity ratios. In general, the green-to-red band ratio of hexagonal phase UCNPs was higher than that of cubic phase UCNPs.25−31 As a result, upon excitation by 980-nm laser, cubic UCNPs show reddish yellow color while hexagonal phase ones are yellowish green (Inserts in Figure 2c and 2d). In order to compare the upconversion efficiency, we measured the luminescence intensity both at the single-particle level and in the solution phase. The single UCNPs dispersed on cover glasses were identified using atomic force microscopy (AFM) (Figure S5 in Supporting Information), and they appear as single spots in the luminescent images taken with the wide-field epi-fluorescence microscope setup as has been studied previously.7 It is shown that the hexagonal core/shell UCNPs are the brightest while cubic core UCNPs are the weakest of the four types (Figure 3a). The average luminescent intensity of each single particle in Figure 3a is summarized in Table S1 in Supporting Information. The temporal single particle luminescence intensity is plotted in Figure 3b. All the types of UCNPs do not exhibit any photoblinking and photobleaching.7,8 The luminescence of hexagonal core/shell UCNPs was approximately 1.6, 20, and 140 times more intense than those of hexagonal core, cubic core/shell, and cubic core UCNPs, respectively. Similar trend was also observed when we measured the photoluminescence of solutions using the same microscope setup (Figure 3c). This is in agreement with the case of bulk upconverting phosphors where the hexagonal phase materials showed much more efficient upconversion efficiency than the cubic phase materials.41,42 We then examined the effect of laser excitation power on the upconversion luminescence. This measurement involved excitation with a wide range of laser power from 0.1 to 2.2 kW/cm2. Figure 4 shows the log-log plots of the luminescence intensity as a function of the laser power 7 ACS Paragon Plus Environment


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for the single particles (Figure 4a) and bulk solutions (Figure 4b) of the four UCNP types (also see Table S2 in Supporting Information). By using proper band-pass filters, we could image and measure the green and red luminescence separately (Figure S6 in Supporting Information). For the cubic phase UCNPs (core and core/shell), the plots were fit linearly with the slopes ranging from 1.4 to 2 indicating two-photon upconversion is operative with a slight saturation in the intermediate photophysical steps. On the other hand, the hexagonal phase UCNPs (core and core/shell) generally displayed slopes with two distinct components.43 In the low power regime (< 0.4 kW/cm2), the slopes were close to 2 while they decreased to 1 in the high power regime (> 0.4 kW/cm2). We also note that the excitation power dependences of relative intensity ratios of the green and red luminescence were quite different among the UCNP types (Figure S7 in Supporting Information). These indicate that the efficiency or the pathway of the upconversion process might be different between the two crystal structures. Further investigation is in progress to clarify this point. For the biomedical applications, UCNPs must be dispersible in aqueous media. To render hydrophobic UCNPs water-dispersible, surface modification with hydrophilic materials should be conducted. Several surface modification methods such as encapsulation with polymers and ligand exchange have been reported. For example, amphiphilic polymers including polyethylene glycol (PEG) derivatives were used for preparing water-dispersible UCNPs. In our previous report, we have shown that the luminescence intensity of water-dispersible cubic phase NaGdF4:Yb3+,Er3+/NaGdF4 UCNPs encapsulated by amphiphilic PEG-phospholipids decreased down to 30% of that of UCNPs dispersed in chloroform.7 To compare the effect of surface ligands on the luminescence efficiency, hexagonal phase NaYF4:Yb3+,Er3+/NaGdF4 UCNPs coated by PEG-phospholipid,7,16,24 PEG-phosphate (PO-PEG),35,36 silica shell (SiO2),9,11,23,37 and 8 ACS Paragon Plus Environment

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mesoporous silica shell (mSiO2)38,44,45 were prepared (Figure S8 in Supporting Information). Table 1 shows the luminescence efficiencies of UCNPs. To preserve upconversion efficiency in aqueous environment, surface coating ligands have to shield UCNPs from surrounding water molecules effectively, because water molecules with high vibrational energy (~3500 cm−1) tend to quench the luminescence intensity as mentioned above.34,35,46 Among the various coating materials, silica shell was the most effective for retaining upconversion luminescence in water (46%) by protecting UCNPs from water molecules. On the other hand, UCNPs coated with mesoporous silica shell, which might be well fit for drug delivery, exhibited slightly decreased luminescence intensity (34%) probably because the water molecules access easily to UCNPs through pores in mesoporous silica. However, we observed significant size increase upon silica or mesoporous silica shell formation (Figure S8 in Supporting Information), which resulted in poor colloidal stability. Ligand exchange on as-synthesized UCNPs with hydrophilic polymers could be an alternative way of producing water-dispersible nanoparticles. Polyacrylic acid has been widely used for preparing UCNPs dispersed in water,17,18,31,39 but the method suffered from low yield of transferring UCNPs from organic to aqueous media as well as significantly reduced upconversion efficiency (27%, data not shown). Ligand exchange of UCNPs by PO-PEG was successful in terms of transfer yield.35 Similar to polyacrylic acid, however, PO-PEG did not effectively shield UCNPs in aqueous environment and upconversion efficiency of PO-PEG coated UCNPs (24%) was lower than those with silica shell (Table 1). For UCNPs encapsulated by PEG-phospholipids, the strong hydrophobic interaction between oleic acid on the particle surface and alkyl chain of the phospholipids form a hydrophobic layer, preventing the access of water molecules to the particle. As a result, upconversion luminescence is better preserved by PEG-phospholipids than by PO-PEG although both coating materials bear PEG moiety. To 9 ACS Paragon Plus Environment


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summarize, among the surface modification schemes tested, we suggest PEG-phospholipids are the most suitable for making UCNPs both bright and stable in aqueous environment. Finally, we conducted live cell imaging using cubic and hexagonal phase core/shell UCNPs to compare their capabilities as biological imaging agent. Both the cubic and hexagonal phase core/shell UCNPs are of similar size and encapsulated by PEG-phospholipids. Prior to their application as cellular imaging probes, the cytotoxic effect of UCNPs was evaluated by a thiazolyl blue tetrazolium bromide (MTT) cell proliferation assay. The results showed that most HeLa cells survived at particle concentrations up to 10 nM (Figure S9 in Supporting Information). For live cell imaging, HeLa cells were incubated with UCNPs (1 nM) for 30 min at 37oC and imaged on the same epi-fluorescence microscope setup as that used for obtaining single-particle images in Figure 3. Figure 5 shows the result of cellular imaging. With exposure time of 1 s, hexagonal phase core/shell UCNPs within HeLa cells showed bright luminescence, while luminescence from cubic phase core/shell UCNPs was barely detected. Only by increasing the exposure time up to 5 s, a dim luminescence image of cubic phase core/shell UCNPs in HeLa cells could be obtained. From this data, it is concluded that hexagonal phase core/shell UCNPs exhibit far better luminescence property than cubic phase ones in bio-imaging applications.

Conclusions In this paper, we compared the efficiency of upconversion photoluminescence with varying crystal structures and surface modification both in the colloidal solutions and at the singleparticle level. It turned out that hexagonal phase core/shell UCNPs exhibited the most intense luminescence among various structures. As for surface modification, PEG-phospholipid was 10 ACS Paragon Plus Environment

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found to be the most effective for preserving both luminescence intensity and dispersibility. From this combination, we could render UCNPs suitable and ideal for biomedical imaging as demonstrated by live-cell imaging. Acknowledgment. T.H. acknowledges the financial support by Korean Ministry of Education, Science and Technology through Institute for Basic Science. Y.D.S. was supported by KRICT (SI-1210), the Nano R&D Program (2011-0019156) of NRF, the Industrial Core Technology Development Program (10037397) of MKE, and the Development of Advanced Scientific Analysis Instrumentation Project of KRISS by MEST. K.T.L. was supported by the Nano R&D Program (2010-0019142), the Pioneer Research Center Program (2011-0002131) of NRF, and the Industrial Core Technology Development Program (10033183) of MKE. H.M.K. was supported by the Public Welfare & Safety Research Program (2011-0020957) of NRF.

Supporting Information Available: Experimental details, schematic presentation of NaREF4 structures, TEM and XRD data of UCNPs, schematic diagram of upconversion processes, AFM and luminescence images of single UCNPs, red-to-green luminescence band ratio data, MTT data of UCNPs, table of single particle intensities, table of slop of power dependence plots, and complete author list of references. This material is available free of charge via the Internet at http://pubs.acs.org.



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43. Wang, Y.; Tu, L.; Zhao, J.; Sun, Y.; Kong, X.; Zhang, H. J. Phys. Chem. C 2009, 113, 7164−7169. 44. Yang, J.; Deng, Y.; Wu, Q.; Zhou, J.; Bao, H.; Li, Q.; Zhang, F.; Li, F.; Tu, Bo.; Zhao, D. Langmuir 2010, 26, 8850−8856. 45. Gai, S.; Yang, P.; Li, C.; Wang, W.; Dai, Y.; Niu, N.; Lin, J. Adv. Funct. Mater. 2010, 20, 1166−1172. 46. Schäfer, H.; Ptacek, P.; Kömpe, K.; Haase, M. Chem. Mater. 2007, 19, 1396−1400.



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Tables

Table 1. Luminescence efficiency of water-dispersed hexagonal phase NaYF4:Yb3+,Er3+/NaGdF4 core/shell UCNPs stabilized by various surface coating materials. The luminescence efficiency was estimated with respect to the luminescence of UCNPs in hexane of the corresponding types obtained with 980-nm excitation of ~200 W/cm2.

coating material

PEG-phospholipid

PO-PEG

SiO2

mSiO2

green (%)

28

20

42

31

red (%)

38

27

49

35

overall (%)

34

24

46

34


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Figure Captions

Figure 1. TEM images of (a) cubic phase NaYF4:Yb3+,Er3+ UCNPs and (b) hexagonal phase NaYF4:Yb3+,Er3+ UCNPs. Upper insets are ED patterns, and lower insets are HRTEM images. XRD patterns of (c) cubic phase NaYF4:Yb3+,Er3+ UCNPs and (d) hexagonal phase NaYF4:Yb3+,Er3+ UCNPs. Bottom line patterns in (c) and (d) are that of cubic (JCPDS 77-2042) and hexagonal phase (JCPDS 16-0334), respectively.

Figure 2. Photoluminescence spectra of four types of UCNP solutions in hexane under 980-nm excitation (~75 W/cm2). (a) cubic and (b) hexagonal phase NaYF4:Yb3+,Er3+ UCNPs. (c) cubic and (d) hexagonal phase NaYF4:Yb3+,Er3+/NaGdF4 core/shell UCNPs. Insets in (c) and (d) are photographs of colloidal solutions in hexane under 980-nm laser irradiation.

Figure 3. (a) Luminescence images of four types of UCNPs (hexagonal core/shell, hexagonal core, cubic core/shell, and cubic core) dispersed on cover glasses under 980-nm irradiation (~2.2 kW/cm2). The exposure time was set to 0.15 s, 0.2 s, 2 s, and 10 s, respectively. (b) Luminescence time traces of single particles presented in Figure 3a under continuous 980-nm excitation for 15 s (0.15-s time bins, ~2.2 kW/cm2). (c) Luminescence time traces of UCNP solutions (20 nM in hexane) under continuous 980-nm excitation for 1.5 s (0.015-s time bins, ~2.2 kW/cm2).

Figure 4. (a) Power dependence of four types of single particles presented in Figure 3a. The luminescence intensity of the cubic core UCNPs was too low to yield reliable power dependence. 17 ACS Paragon Plus Environment


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(b) Power dependence of four types of UCNP solutions.

Figure 5. Cellular uptake of cubic and hexagonal phase NaYF4:Yb3+,Er3+/NaGdF4 core/shell UCNPs in a single living HeLa cell. Bright-field images of the cell (left panels), luminescence images (middle panels), and overlay images (right panels) are shown. (a) cubic phase UCNPs, 5 s of exposure time. (b) hexagonal phase UCNPs, 1 s of exposure time.


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Figure 1.



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Figure 2.


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Figure 3.



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Figure 4.


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Figure 5.



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Table of Contents Image


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