Yb3+,Er3+ Multifunctional Hybrid

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The Journal of Physical Chemistry

Au Nanorods@NaGdF4:Yb3+,Er3+ Multifunctional Hybrid Nanocomposites with Upconversion Luminescence, Magnetism and Photothermal Property

Yan Song,a Guixia Liu,*a Xiangting Dong,a Jinxian Wang,a Wensheng Yu,a and Jingmei Li,b

a

Key Laboratory of Applied Chemistry and Nanotechnology at Universities of Jilin

Province, Changchun University of Science and Technology, Changchun 130022, P. R. China. Tel.:+86-431-85582574. Fax.: +86-431-85383815; E-mail address: [email protected] b

School of Life Science, Changchun University of Science and Technology,

Changchun 130022, P. R. China

* Corresponding author. Tel.:+86-431-85582574. Fax.: +86-431-85383815 E-mail address: [email protected]

ACS Paragon Plus Environment


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Abstract: Multifunctional nanomaterials possessing upconversion luminescence, magnetic and photothermal properties show promising applications in biology and medicine. In this study, a new kind of luminescent-magnetic-thermal core-shell hybrid nanocomposites was fabricated combining rare earth Yb3+ and Er3+ ions doped NaGdF4 nanocrystals as shell layer materials and gold nanorods (AuNRs) as cores. The structure, morphology, composition and properties of the multifunctional hybrid nanocomposites were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), upconversion photoluminescence (UCPL) spectra, vibrating sample magnetometer (VSM), sensitive thermometer and cytotoxicity assessment, respectively. The multifunctional hybrid nanocomposites have rod-like morphology and core-shell structure. The uniform NaGdF4:Yb3+,Er3+ shell with a thickness of around 4.5 nm was coated on the surface of AuNRs with a length of 40 nm and a diameter of 12 nm. The AuNRs@NaGdF4:Yb3+,Er3+ multifunctional nanocomposites provide an excellent upconversion emission under excitation at 980 nm and a super-paramagnetic behaviour with magnetic susceptibility of 8.0×10−5 emu·g-1·Oe-1 at 300 K and saturation magnetization value of 102 emu·g-1 at 2 K. More significantly, when AuNRs@NaGdF4:Yb3+,Er3+ aqueous suspensions of 100 µg·mL-1 were irradiated by a 980 nm NIR laser for 10 min, the temperature can be significantly elevated to 48 °C. At the same time, the temperature of photothermal transduction can be easily controlled

by adjusting

the concentration of

nanocomposites.

Preliminary

investigation of incubating with HeLa cells displays that the multifunctional nanocomposites exhibit a good biocompatibility. Moreover, the nanocomposites may be effectively utilized for bioimaging and photothermal therapy in living cells.


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1 Introduction Lanthanide doped upconversion nanocrystals have been of considerable research interest in recent years due to their unique optical properties and attractive potential applications1. As luminescence material, lanthanide doped nanomaterials have obvious advantages, including low toxicity, narrow emission peaks, long emission lifetimes, higher photochemical stability and quantum yields compared with conventional semiconductor quantum dots and organic dyes2,3. More importantly, the lanthanide doped upconversion nanomaterials can efficiently convert near-infrared radiation into visible light4,5. The near infrared (NIR) excitation wavelength lies in an optically transparent window for biological tissues, promising low radiation damage and deeper penetration depth for vivo applications6. Therefore, upconversion luminescence nanocrystals are promising new generation of luminescence materials for optical imaging of deeper tissues in the fields of biology and biomedicine7,8. The lanthanide doped fluorides as excellent luminescent rigid host matrixes have been widely studied in the current work owing to their low phonon energy, high luminous efficiency and excellent chemical stability9,10. Among the lanthanide doped fluorides upconversion materials, hexagonal-phase NaGdF4 nanocrystals act as outstanding host materials for achieving highly bright upconversion luminescence. Furthermore, Gd-based fluoride luminescent nanocrystals are very suitable for creating multifunctional materials owing to their unusual magnetic and optical properties11,12. They have been recently emerged as optical imaging nanoprobes and magnetic resonance image contrast agents for magnetic/optical bimodal detection13-16 The upconversion luminescence is usually studied by using the trivalent Er3+ ions as activators because the metastable levels 4I9/2 and 4I11/2 can be conveniently populated17. To greatly enhance the upconversion luminescence of the Er3+ activators, the trivalent



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Yb3+ ions are commonly used as effective sensitizing center and give an efficient excitation band around 980 nm18,19. Thus, the NaGdF4 host lattice doped with lanthanide combinations of Yb3+ and Er3+ has been considered as a bifunctional material combining with upconversion emission and magnetic properties. The unique optical and interesting plasmonic properties of noble metal nanoparticles have attracted significant attention in recent years20-22. In particularly, gold nanorods (AuNRs) as the popular plasmonic nanomaterials have generated interest for the application in the biomedical fields, including biochemical sensing, plasmon enhanced spectroscopies, cell imaging and photothermal therapy for cancer23-26. Compared to spherical gold nanoparticles, AuNRs offer significant advantages for their two distinct plasmon modes associated with the longitudinal localized surface plasmon resonance (LSPR) mode and the transverse LSPR mode, which will exhibit strong surface plasmon absorption in the NIR-to-IR region27. Under the resonant excitation, the free electrons will collectively oscillate along the length axis and the transverse direction of the AuNRs23,28. The electron oscillations can significantly alter the electromagnetic field near the nanorod surface. The effect leads to surface enhanced Raman scattering29, fluorescence enhancements and quenching surrounding the nanorod which depend on the spacing distance of the donor-acceptor pairs30-32, as well as the spectral overlap of their emission and absorption wavelengths33,34. Surface plasmon resonance of gold nanocrystals has been reported to achieve photoluminescence

enhancement

or

quenching

from

lanthanide

doped

nanomaterials35-37. The increase of the excitation rate or radiative decay rate will result in the enhancement of upconversion luminescence intensity38-40. When luminescence materials are located in very close proximity to the gold surface, the


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luminescence intensity is reduced dramatically due to the increase of their non-radiative decay rate and non-radiative energy transfer41-43. Integration of Gd-based upconversion luminescent materials with AuNRs could construct a luminescence resonance energy transfer (LRET) system using luminescent nanocrystals as the donors and AuNRs as the acceptors. In addition, the designed LRET system can possess simultaneously upconversion luminescence, magnetic and photothermal property. We are interested in developing a multifunctional hybrid nanocomposite for widening the scope of applications. The multifunctional hybrid nanocomposites are composed of a nanoshell layer of upconversion luminescence nanocrystals and a core of AuNRs (Figure 1a).

Figure 1 Schematic illustration of the structure (a) and properties (b) for multifunctional hybrid nanocomposites.

The upconversion luminescence, magnetism and photothermal transduction of the multifunctional hybrid nanocomposites are illustrated, as shown in Figure 1b. In the work, the upconversion emission from Er3+ ions and the magnetic property from Gd3+ are provided in the multifunctional hybrid nanocomposites. In addition, the energy transfer may be occurred from the NaGdF4:Yb3+,Er3+ nanocrystals to AuNRs in the



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multifunctional hybrid nanocomposites based on the efficient overlap of spectra. Furthermore, the LSPR effects from AuNRs could lead to strong NIR light adsorption in the NIR-to-IR region and photothermal transduction. Finally, the biocompatibility of the nanocomposites, upconversion luminescence imaging in vitro and the photothermal effect for the cancer cells have been studied.

2 Experimental section 2.1 Materials Gadolinium oxide (Gd2O3, 99.99%), ytterbium oxide (Yb2O3, 99.99%), erbium oxide (Er2O3, 99.99%), ammonium fluoride (NH4F), sodium borohydride (NaBH4), Silver nitrate (AgNO3), ascorbic acid (AA), sodium citrate, absolute alcohol (ethanol) and dimethylsulfoxide (DMSO) were purchased from Sinopharm Chemical Reagent Co., Ltd. Gold chloride tetrahydrate (HAuCl4) and hexadecyltrimethylammonium bromide (CTAB) were purchased from Aladdin. Deionized (DI) water was used in all experiments. Rare earth nitrate stock solutions were obtained by dissolving the corresponding rare earth oxides in dilute HNO3 under heating with agitation followed by evaporating the solvent.

2.2 Synthesis of AuNRs The AuNRs were synthesized by the previously reported procedures with slight modification44. In the typical procedures of preparing AuNRs, the Au seeds with CTAB-stabilized were first synthesized. A 0.5 mL amount of 5 mM HAuCl4 aqueous solution was mixed with 5 mL of 0.2 mol·L-1 CTAB solution by gentle inversion. Then, 0.6 mL of a freshly prepared ice-cold aqueous 0.01 mol·L-1 NaBH4 solution was diluted to 1 mL with water and was injected to the mixture solution. The mixture


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solution color immediately changed from yellow to pale brown. The seed solution was left undisturbed at room temperature for 30 min to age before use. After seed generation, AuNRs were prepared by a seed-mediated method. To prepare the growth solution, aqueous solutions of HAuCl4 (4 mL, 5 mol·L-1), AgNO3 (0.24 mL, 10 mol·L-1) and a small amount of HCl (37 wt % in water, 12.1 mol·L-1) were added into an aqueous CTAB solution (25 mL, 0.1 mol·L-1) in a beaker flask. Then a freshly prepared ascorbic acid solution (0.17 mL, 0.1 mol·L-1) was added. After the solution was mixed by inversion, the growth solution became colorless after addition with AA. Finally, 100 µL of the seed solution was injected into the growth solution, the resulting growth solution was gently vibrated for 30 s and left undisturbed in a water bath for 12 h at 27 °C.

2.3 Synthesis of the multifunctional hybrid nanocomposites The 5 mL of an aqueous solution of CTAB-modified AuNRs was diluted with DI water to 20 mL. Subsequently, the gold colloidal solution was mixed with a sodium citrate solution (0.25 mmol) and magnetic stirred for 30 min. Then, a total amount of 0.25 mmol of Gd(NO3)3, Yb(NO3)3 and Er(NO3)3 aqueous solutions (molar ratio Gd : Yb : Er = 39: 10 : 1) were added into the above solutions at room temperature. After continuously stirring for 30 min, 0.24 mmol NH4F was injected into the above solution as F− source. The mixture solution was kept stirring at room temperature for 1 h and then transferred into a Teflon-lined stainless steel autoclave with 50 mL capacity, sealed and maintained at 180 °C for 3 h. As the autoclave was cooled to room temperature naturally, the reaction products were collected by centrifugation at 8000 rpm for 5min. The precipitates were washed at least twice with DI water and ethanol for several times, dried in air at 60 °C for 12 h.



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2.4 Cytotoxicity assessment The cytotoxicity was evaluated by performing methyl thiazolyl tetrazolium (MTT) assay of the HeLa cells incubated with the particles. Cells were first seeded into a 96-well plate with a density of 1×104 cells/well and cultured for 24 h under a humidified atmosphere of 5% CO2 at 37 °C. The cells were treated with different concentrations (0.05, 0.10, 0.20, 0.50, 1.00 and 2.00 mg·mL-1) of nanocomposites and maintained for 24 h at 37 °C under 5% CO2 incubator. The MTT solution (20 µL, 5 mg·mL-1) was then added to each well of the plate and cultured for another 4 h at 37 °C under 5% CO2. Afterwards, the MTT solution was removed and dimethylsulfoxide (DMSO) (150 µL) was added to each well. The optical density (OD) of the suspension was monitored with an absorbance microplate reader TQuant (BioTEK, USA) at a wavelength of 490 nm. The cumulative analysis of 3 wells per group was recorded. The cell viability was calculated by the following equation: Cell viability (%) =[OD]test/[OD]control×100%. Here, [OD]test denotes absorbance of test wells treated with multifunctional hybrid nanocomposites, while [OD]control denotes absorbance of untreated control wells.

2.5 Upconversion luminescence imaging of living cells In vitro upconversion luminescence imaging of HeLa cells was performed with a confocal laser scanning microscope (Nikon Ti-S) under the excitation of an external 980 nm laser. The Hela cells were first seeded into in 6-well culture plates with a density of 1×104 cells/well and cultured for 24 h at 37 °C under a humidified atmosphere of 5% CO2. Then, the cells were incubated with 0.10 mg·mL-1 of AuNRs@NaGdF4:Yb3+,Er3+ nanocomposites at 37 °C. After 6 h of incubation time, the cells were washed with phosphate buffered saline (PBS). The bright-ﬁeld image


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and upconversion luminescence image of HeLa cells were obtained.

2.6 Photothermal therapy of living cells The Hela cells were first seeded into a 96-well plate with a density of 1×104 cells/well and cultured for 24 h under a humidified atmosphere of 5% CO2 at 37 °C. Then, the cells were treated by a culture medium with 0.10 mg·mL-1 of AuNRs@NaGdF4:Yb3+,Er3+ nanocomposites. After 6 h of incubation time, the cells were washed with PBS, and the wells were irradiated with a 980 nm NIR laser for different time (1, 2, 4, 8, 10 min). The cell viability was calculated by MTT assay, which were used to evaluate the photothermal effect of AuNRs@NaGdF4:Yb3+,Er3+ multifunctional nanocomposites.

2.7 Characterizations The X-ray diffraction (XRD) patterns of the as-synthesized samples were obtained on a Bruker D8 FOCUS diffractometer in the 2θ range from 10° to 90° (Cu Kα radiation, λ= 1.54056 Å). Transmission electron microscopy (TEM) images were acquired on a JEOL JEM-2010 transmission electron microscope under a working voltage of 200 kV. The scanning transmission electron microscopy (STEM) image was

investigated

using

JEOL-STEM

scanning

system

(ASID-2000).

The

energy-dispersive spectroscopic (EDS) analysis was performed using an Oxford INCA energy system operated at 200 kV. UV-Vis absorption measurements were carried

out

on

Shimadzu

UV-2450

spectrophotometer.

The

upconversion

luminescence properties of the samples were recorded on Hitachi F-7000 spectrophotometer at room temperature equipped with a 980 nm diode laser as the excitation source. The magnetic performance of samples was measured by a vibrating sample magnetometer (VSM, MPMS SQUID XL). The sample was firstly dispersed



into water to form the aqueous suspension. Then, 2 ml of the aqueous suspension was added into the quartz cuvette and exposed to a 980 nm NIR laser irradiation. The aqueous solution temperature was recorded using a HT3500C sensitive thermometer.

3 Results and discussion 3.1 Structure of the multifunctional nanocomposites △ ☆

△ △

c

△

NaGdF4 Au

△

☆ △

△

(200)

(111)

△

Intensity (a.u.)

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△

☆△

b a JCPDS No.65-2870 JCPDS No.27-0699

10

20

30

40

50

60

2-Theta (degree)

Figure

2

XRD

patterns

of

AuNRs

(a),

NaGdF4:Yb3+,Er3+

nanocrystals

(b)

and

AuNRs@NaGdF4:Yb3+,Er3+ nanocomposites (c). The standard card of β-NaGdF4 (PDF: 27-0699) and Au (PDF: 65-2870) were given as reference.

Figure 2 shows the XRD patterns of the AuNRs, NaGdF4:Yb3+,Er3+ nanocrystals and AuNRs@NaGdF4:Yb3+,Er3+ nanocomposites, which provide direct evidence for the formation and crystal structure of the as-prepared samples. As shown in Figure 2a, the peaks of Au coincide well with the standard card (JCPDS card 65-2870) of the face-centered cubic phase of Au. For the NaGdF4:Yb3+,Er3+ nanocrystals (Figure 2b), all the diffraction peaks can be easily indexed to the data in the standard card of pure hexagonal-phase NaGdF4 crystal (JCPDS No. 27-0699) and no diffraction peaks of any other phases or impurities are detected. The result indicates that pure


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hexagonal-phase NaGdF4 can be synthesized under this condition. The XRD pattern of the AuNRs@NaGdF4:Yb3+,Er3+ nanocomposites, as illustrated in Figure 2c, indicates a hybrid composed of NaGdF4 and Au. The main strong diffraction peaks match well with the standard hexagonal-phase NaGdF4 and two tiny diffraction peaks at 38.3° and 43.2° are consistent with the (111), (200) lattice planes of the standard X-ray data for the face-centered cubic phase of Au (JCPDS No. 65-2870), respectively.

3.2 Morphology and composition of the multifunctional nanocomposites

Figure 3 TEM images of AuNRs (a) and AuNRs@NaGdF4:Yb3+,Er3+(b), EDS spectrum of AuNRs@NaGdF4:Yb3+,Er3+(c),

STEM

image

and

the

AuNRs@NaGdF4:Yb3+,Er3+(d).


elemental

mapping

of


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As show in Figure 3a, the morphology and size of the as-prepared AuNRs were observed by means of TEM. It can be seen that the AuNRs with rod-like morphology of 40 nm in length and 12 nm in diameter were synthesized using seed-mediated growth approach. The TEM image in Figure 3b shows that the as-synthesized AuNRs@NaGdF4:Yb3+,Er3+ composites have rod morphology and core-shell structure with uniform NaGdF4:Yb3+,Er3+ coatings on the surface of AuNRs, the thickness of NaGdF4:Yb3+,Er3+

layer

is

around

4.5

nm.

The

composition

of

AuNRs@NaGdF4:Yb3+,Er3+ was further confirmed by EDS analysis. In the EDS spectrum (Figure 3c), the peaks of Na, Gd, F, Yb and Er elements from shell layers are also detected in AuNRs@NaGdF4:Yb3+,Er3+ nanocomposites, beside of the peaks of Au element from cores. In Figure 3d, the scanning transmission electron microscopy (STEM) image clearly shows a significant difference contrast between core and shell, which demonstrated the core-shell structure. In the element mapping of a single AuNRs@NaGdF4:Yb3+,Er3+ nanocomposite, the Au, F, Na, Gd, Yb, and Er element were observed. The result of element mapping further confirms that the core-shell structures of the AuNRs@NaGdF4:Yb3+,Er3+ are achieved, and the AuNRs located in the core layer are surrounded by the NaGdF4:Yb3+,Er3+ nanoshells.

3.3 Photoluminescence properties of the multifunctional nanocomposites It is known that the AuNRs possess two SPR modes associated with the longitudinal mode and the transverse mode, which correspond to free electrons collective oscillations along the length axis and the transverse direction of the nanorod. In order to examine the effect of SPR modes on upconversion luminescence, the optical properties of the fabricated AuNRs@NaGdF4:Yb3+,Er3+ nanocomposites were investigated by the UV-Vis absorption spectrum and upconversion emission spectra. Figure 4a shows the UV-Vis absorption spectrum of AuNRs in DI water, there are two


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SPR absorption bands around 525 nm and 790 nm, which are attributed to the transverse absorption and longitudinal absorption of AuNRs, respectively. From the upconversion emission spectrum of NaGdF4:Yb3+,Er3+ under 980 nm light excitation (Figure 4b), three main emission bands in the visible region: the green emissions at 520 nm and 540 nm corresponding to the 2H11/2-4I15/2, 4S3/2-4I15/2 transitions and the red emission at 652 nm ascribing to the 4F9/2-4I15/2 transitions of Er3+ ions. From Figure 4a and 4b, it can be seen clearly that the transverse SPR absorption band of AuNRs overlaps well with the green emissions of Er3+ ions in the NaGdF4:Yb3+,Er3+, which is satisfying the prerequisite for efficient LRET. In the LRET system, the NaGdF4:Yb3+,Er3+ upconversion luminescence nanocrystals are very suitable for using as energy donors and the AuNRs are suitable for using as energy acceptors. As shown in Figure 4b, we compare the upconversion luminescence intensity of the NaGdF4:Yb3+,Er3+ nanocrystals in absence and presence of AuNRs under the excitation of 980 nm laser. Upon addition of AuNRs, the upconversion emission intensities at 520 nm (2H11/2-4I15/2 transition) and 540 nm (4S3/2-4I15/2 transition) in the NaGdF4:Yb3+,Er3+ nanocrystals decrease dramatically, which indicates that high probability of LRET between NaGdF4:Yb3+,Er3+ nanocrystals and AuNRs. However, the red emission at 652 nm originated from the 4F9/2-4I15/2 transitions of Er3+ ions cannot be clearly quenched by AuNRs, thereby ruling out the decrease of the excitation rate due to the longitudinal absorption of AuNRs at 790 nm. The above results suggest that the non-radiative LRET occurs from NaGdF4:Yb3+,Er3+ upconversion luminescence nanocrystals to AuNRs. As shown in the inset of Figure 4b, the NaGdF4:Yb3+,Er3+ display obvious green emission under the excitation of 980 nm laser with a power of 1.2 W, and a yellow-green emission of the AuNRs@NaGdF4:Yb3+,Er3+ nanocomposites can be observed due to non-radiative



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LRET between NaGdF4:Yb3+,Er3+ nanocrystals and AuNRs. Figure 4c shows the upconversion emission spectra of AuNRs@NaGdF4:Yb3+,Er3+ nanocomposites at different pumping powers under a 980 nm NIR laser excitation. Two emissions in the green region and one emission in the red region are displayed in each upconversion emission spectra. More significantly, the upconversion emissions intensities grow at different rates with increasing of pump power. In order to define the number of photons involved in the upconversion process, the dependence of the upconversion emissions intensity versus the pump power was investigated. Figure 4d shows the Log-Log plots of the upconversion emissions intensities as a function of the excitation pump power, in which the slope indicates the number of photons involved in the upconversion process. The slope values of the fitted line of Log-Log plots for the upconversion emissions at 520, 540 and 652 nm are 1.93, 1.62 and 1.86, respectively. The pump power dependence analysis illustrates that two photons are involved to produce the green and red emissions in the upconversion process.


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Figure 4 a: UV-Vis absorption spectrum of AuNRs; b: upconversion emission spectra for NaGdF4:Yb3+,Er3+ before and after decorating with AuNRs (the inset shows the photograph of the NaGdF4:Yb3+,Er3+ (i) and AuNRs@NaGdF4:Yb3+,Er3+ (ii) without (left) and with (right) 980 nm laser excitation); c: the upconversion emission spectra of the different pumping powers under 980 nm excitation; d: Log–Log plots of the upconversion emission intensity versus pump power for AuNRs@NaGdF4:Yb3+,Er3+. a

1

3+

3+

τ=0.347 ms

NaGdF4:Yb ,Er

3+

3+

τ=0.164 ms

Log intensity (a.u.)

AuNRs@NaGdF4:Yb ,Er 4

4

3+

S3/2 → I15/2 Er

0.1

0.01 0

1

2

Time (ms)

b

1

3+

3+

NaGdF4:Yb ,Er

τ=0.423 ms 3+

3+

AuNRs@NaGdF4:Yb ,Er

Log intensity (a.u.)

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4

τ=0.376 ms 4

3+

F9/2 → I15/2 Er

0.1

0.01 0

1

2

Time (ms)

Figure 5 Upconversion luminescence decay curves of Er3+ 4S3/2→4I15/2 transition(540 nm) (a) and 4

F9/2→4I15/2 transitions(652 nm) (b) in the NaGdF4:Yb3+,Er3+ and AuNRs@NaGdF4:Yb3+,Er3+.

To further clarify the effect of AuNRs in the LRET system, the decay curves of 4

S3/2→4I15/2 transition(540 nm) and 4F9/2→4I15/2 transitions(652 nm) of Er3+ ions in

NaGdF4:Yb3+,Er3+ and AuNRs@NaGdF4:Yb3+,Er3+ were investigated under excitation



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by a 980 nm NIR laser. As can be seen from Figure 5a and b, both decay curves of Er3+ ions can be best fitted with a single exponential function of I=I0+Aexp(-x/τ) (τ corresponds to the lifetime of Er3+). The decay times of Er3+ ions in NaGdF4:Yb3+,Er3+ and AuNRs@NaGdF4:Yb3+,Er3+ were calculated. The result shows that the decay times of 4F9/2→4I15/2 transitions of Er3+ ions at 652 nm are 0.423 and 0.376 ms for NaGdF4:Yb3+,Er3+ and AuNRs@NaGdF4:Yb3+,Er3+, respectively. However, for the 4S3/2→4I15/2 transition of Er3+ ions at 540 nm, the decay times, that is the radiative lifetimes decrease from 0.347 to 0.164 ms due to introducing AuNRs into the LRET system, which indicate that the non-radiative LRET occurs in the AuNRs@NaGdF4:Yb3+,Er3+ nanocomposites.

Figure 6 Schematic of possible upconvesion population of Yb3+, Er3+ ions and energy transfer process with AuNRs.

The possible upconversion mechanism of NaGdF4:Yb3+,Er3+ nanocrystals as well as the energy transfer process between NaGdF4:Yb3+,Er3+ and AuNRs are presented in Figure 6. In this mechanism, the Yb3+ ions are excited to the 2F5/2 level from the ground state 2F7/2 level by continuously absorbing the 980 nm photons and transfer energy to the 4I11/2, 4F9/2 and 4F7/2 level of Er3+ ions. The Er3+ ions are excited first


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from the ground state 4I15/2 level to the 4I11/2 level and then from 4I11/2 to 4F7/2 level via a two photons process. Subsequently, the electrons in the 4F7/2 level of Er3+ ions decay non-radiatively to the 2H11/2, 4S3/2, or 4F9/2 level and the green and red emissions occur. The NaGdF4:Yb3+,Er3+ nanocrystals and the AuNRs are close enough and possess the efficient spectral overlap in the AuNRs@NaGdF4:Yb3+,Er3+ nanocomposites, which can result in quenching of luminescence. Thus, the green luminescence emission of the Er3+ ions in AuNRs@NaGdF4:Yb3+,Er3+ nanocrystals decreases dramatically and the energy transfers from NaGdF4:Yb3+,Er3+ to AuNRs. AuNRs as energy acceptors can convert energy to heat and the heat will greatly be useful in photothermal therapy.

3.4 Magnetic properties of the multifunctional nanocomposites The magnetic properties of the as-prepared samples were investigated with a VSM magnetometer. Figure 7a shows the hysteresis loops of pure NaGdF4:Yb3+,Er3+ and AuNRs@NaGdF4:Yb3+,Er3+ nanocomposites at room temperature (300 K). From the magnetic hysteresis loops of NaGdF4:Yb3+,Er3+ nanocrystals, we can see that the NaGdF4:Yb3+,Er3+ nanocrystals exhibit a magnetic susceptibility of 9.0×10−5 emu·g-1·Oe-1, and have a good super-paramagnetism in the magnetic range of -20 kOe to 20 kOe at 300 K due to no coercivity or remanence. The magnetic hysteresis loop of AuNRs@NaGdF4:Yb3+,Er3+ nanocomposites shows that the magnetization is around 1.6 emu·g-1 at 20 kOe, which is smaller than the magnetization of pure NaGdF4:Yb3+,Er3+ nanocrystals. The lower measured magnetization can be attributed to the addition of AuNRs. As shown in Figure 7b, the magnetic hysteresis loops for the samples at low temperature (2 K) were achieved, which exhibit that the pure NaGdF4:Yb3+,Er3+ presents obvious paramagnetic with saturation magnetization value of 115 emu·g-1. The saturation magnetization value of the nanocomposites is reduced to 102 emu·g-1 because of the presence of AuNRs. Compared with the pure



NaGdF4:Yb3+,Er3+ nanocrystals, the saturation

magnetization

value

of the

AuNRs@NaGdF4:Yb3+,Er3+ nanocomposites is obviously decreased due to the introduction of the diamagnetic contribution of AuNRs, which results in low mass fraction of the NaGdF4:Yb3+,Er3+ magnetic substance in the nanocomposites. In addition, the coercivity and remanence of the AuNRs@NaGdF4:Yb3+,Er3+ nanocomposites are close to zero, indicating that nanocomposites present a super-paramagnetic behaviour. It is expected that the AuNRs@NaGdF4:Yb3+,Er3+ nanocomposites with a good super-paramagnetic behaviour have crucial applications value in biomedicine and biotechnology.

a

2

3+

3+

NaGdF4:Yb ,Er

3+

3+

Magnetization (emu·g-1)

AuNRs@NaGdF4:Yb ,Er

1

T= 300 K

0

-1

-2 -20

-15

-10

-5

0

5

10

15

20

10

15

20

Magnetic field (kOe)

b

3+

3+

NaGdF4:Yb ,Er 100

Magnetization (emu·g-1)

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 30

3+

3+

AuNRs@NaGdF4:Yb ,Er T= 2 K

50

0

-50

-100

-20

-15

-10

-5

0

5

Magnetic field (kOe)

Figure 7 Magnetic hysteresis loops of NaGdF4:Yb3+,Er3+ in the absence and presence of AuNRs at 300 K(a) and 2 K(b).


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3.5 photothermal transduction properties of the multifunctional nanocomposites

50

a

AuNRs@NaGdF4:Yb3+,Er3+ NaGdF4:Yb3+,Er3+

Temperature (°C)

45

water

40 35 30 25 20 0

50

2

4

6

8

Time (min)

10

b

40

Temperature (°C)

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


30

20

10

0 20

40

60

80

100

Concentration of Au@NaGdF4:Yb3+,Er3+ (µg·ml-1)

Figure

8

(a)

Temperature

change

curves

of

water,

NaGdF4:Yb3+Er3+

and

AuNRs@NaGdF4:Yb3+Er3+ nanocomposites under 980 nm laser irradiation at 1.2 W·cm-2; (b) photothermal effect of AuNRs@NaGdF4:Yb3+Er3+ nanocomposites with different concentrations under 1.2 W·cm-2 980 nm laser irradiation for 10 min.

The strong surface plasmon absorption of AuNRs motivated us to investigate its photothermal

properties.

We

examined

the

temperature

change

of

AuNRs@NaGdF4:Yb3+,Er3+ aqueous suspensions with a 980 nm NIR laser at power density of 1.2 W·cm-2 irradiation for 10 min. As depicted in Figure 8a, the



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

temperature of the water in the absence of AuNRs@NaGdF4:Yb3+,Er3+ increases only 8 °C under 980 nm laser irradiation for 10 min. In comparison, the aqueous suspension temperature of AuNRs@NaGdF4:Yb3+,Er3+ and NaGdF4:Yb3+,Er3+ can be significantly elevated during 980 nm laser irradiation at power density of 1.2 W·cm-2, and increase with the extension of irradiation time. Irradiating to 100µg·mL-1 AuNRs@NaGdF4:Yb3+,Er3+ aqueous suspensions at 10 min, the temperature increases to 48 °C, while the temperature of 100 µg·mL-1 NaGdF4:Yb3+,Er3+ aqueous suspensions reaches 32 °C. The result suggests that AuNRs@NaGdF4:Yb3+,Er3+ nanocomposites have a prominent photothermal transduction property and the temperature increment is resulted mainly from AuNRs and slightly from NaGdF4:Yb3+,Er3+.

As

the

concentration

of

AuNRs@NaGdF4:Yb3+,Er3+

nanocomposites increases, an obvious temperature increment from 34.0 °C to 49.3 °C is also found (Figure 8b), confirming that the temperature of photothermal transduction can be easily controlled by adjusting the concentration of AuNRs@NaGdF4:Yb3+,Er3+ nanocomposites. The above results indicate that the AuNRs@NaGdF4:Yb3+,Er3+ nanocomposites could offer potential application in biomedical fields as an efficient photothermal transduction agent.

3.6 Cytotoxicity assay of the multifunctional nanocomposites A good biocompatibility of nanomaterials is an essential prerequisite and crucial factor for biomedical applications. In order to investigate the potential biocompatibility of the multifunction nanocomposites, the MTT-based cell viability assays were performed on HeLa Cells. Figure 9 shows the cell viability of the cells incubated with AuNRs@NaGdF4:Yb3+,Er3+ at different concentrations compared with the cells in the untreated control wells. It is obviously shown a relatively small variation in cell viability even at high concentrations of the nanoparticles after


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Page 21 of 30

incubation for 24 h with AuNRs@NaGdF4:Yb3+,Er3+ at different concentrations ranged from 0.1 to 2.0 mg·mL-1. Moreover, the cell viability is more than 80% even incubation for 24 h at the concentration of 2.0 mg·mL-1. The MTT assay results indicate that the AuNRs@NaGdF4:Yb3+,Er3+ nanocomposites have low cytotoxicity, and can be applied in biological fields.

100

80

Cell viability (%)

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


60

40

20

0 0

0.05

0.1

0.2

0.5

1.0 3+

3+

2.0 -1

Concentration of AuNRs@NaGdF4:Yb ,Er (mg⋅mL ) Figure 9 Cell viability data of HeLa cells after incubation with the AuNRs@NaGdF4:Yb3+,Er3+ conjugates with different concentrations for 24 h.

3.7 In vitro upconversion luminescence imaging As an outstanding upconversion luminescence material, NaGdF4:Yb3+,Er3+ nanomaterials have been useful for the bioimaging in living cells11. In order to examine the potential of AuNRs@NaGdF4:Yb3+,Er3+ nanocomposites in cells imaging, the living cells were incubated for 12 h with the nanocomposites at the concentration of 0.5 mg·mL-1 and the upconversion luminescence imaging in vitro were carried out by utilizing confocal laser scanning microscopy with a 980 nm NIR laser. As presented in Figure 10, the upconversion luminescence imaging of Hela cells was



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verified. The strong upconversion luminescence signals were observed in the HeLa cells region of the bright-field. The bright-field and upconversion luminescence images of Hela cells were generally merged, which confirms that the upconversion luminescence signals were localized in the intracellular region. Therefore, the results suggest that the AuNRs@NaGdF4:Yb3+,Er3+ nanocomposites is a successful image agent and can be used for the upconversion luminescence imaging of living cells.

a

b

c

Figure 10 Confocal laser scanning microscopy images of HeLa cells after incubation for 6 h at 37 °C with AuNRs@NaGdF4:Yb3+,Er3+ under excitation of a 980 nm laser (a) bright-field image, (b) upconversion luminescence image and (c) overlay of bright-field and upconversion luminescence image.

3.8 Photothermal effect of the multifunctional nanocomposites The photothermal therapy efficacy of the nanocomposites on cancer cells in vitro was investigated based on the prominent photothermal transduction property of AuNRs@NaGdF4:Yb3+,Er3+ nanocomposites. The Hela cells were irradiated by using a 980 nm laser after incubation with 100 µg·mL-1 samples at 37 °C for 12 h. Then, the cell viability was determined by the MTT assay, as shown in Figure 11. It is obvious that

the

cell

viability

in

the

absence

of


NaGdF4:Yb3+,Er3+

and

Page 23 of 30

AuNRs@NaGdF4:Yb3+,Er3+ is more than 95% under 980 nm laser irradiation for 10 min. In presence of NaGdF4:Yb3+,Er3+, the viability of the cells decreases 20% under irradiation of 980 nm NIR laser for 10 min. However, the viability of the cells incubated with AuNRs@NaGdF4:Yb3+,Er3+ nanocomposites is reduced with an increase of the 980 nm NIR laser irradiation time, and the Hela cells of more than 50% were killed under the 980 nm NIR laser irradiation for 10 min due to photothermal effect of the AuNRs@NaGdF4:Yb3+,Er3+ nanocomposites. The results demonstrate that the AuNRs@NaGdF4:Yb3+,Er3+ nanocomposites may be a promising photothermal agent for cancer cells therapy.

100

Cell viability (%)

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


80

Control NaGdF4:Yb3+,Er3+

60

3+

3+

AuNRs@NaGdF4:Yb ,Er 40 0

1

2

4

8

10

Time (min) Figure 11 Cell viability data of HeLa cells incubated with 100 µg·mL-1 of the NaGdF4:Yb3+,Er3+ and AuNRs@NaGdF4:Yb3+,Er3+ under a 980 nm laser irradiation for different time.

4 Conclusions In summary, novel multifunctional hybrid nanocomposites with upconversion luminescence, magnetism and photothermal transduction were successfully developed based on combination of NaGdF4:Yb3+,Er3+ and AuNRs. The hybrid nanocomposites



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

with rod like morphology were core-shell structure. A shell layer of NaGdF4:Yb3+,Er3+ with a thickness of around 4.5 nm was formed on the surface of the AuNRs with 40 nm of length and 12 nm of diameter in the multifunctional hybrid nanocomposites. Compared with the upconversion emission spectrum of NaGdF4:Yb3+,Er3+ nanocrystals, the green emission of AuNRs@NaGdF4:Yb3+,Er3+ was decreased dramatically due to the LRET process between NaGdF4:Yb3+,Er3+ nanocrystals and AuNRs. Meanwhile, the multifunctional hybrid nanocomposites exhibited a good super-paramagnetism in the magnetic range of -20 kOe to 20 kOe at 300 K and 2 K. More importantly, the surface plasmon resonance adsorption from AuNRs could lead to strong photothermal transduction under irradiation of a continuous-wave 980 nm laser. In addition, nanocomposites have a good biocompatibility, signifying suitable for

biomedical

application.

Meanwhile,

the

AuNRs@NaGdF4:Yb3+,Er3+

nanocomposites can be successfully utilized for upconversion luminescence imaging of living cells and photothermal therapy of cancer cells. We envision that a new versatile platform for biological applications can be provided from upconversion luminescence, magnetism and photothermal transduction of multifunctional hybrid nanocomposites.

Acknowledgements This work was financially supported by the National Natural Science Foundation of P.R. China (NSFC) (Grant No. 51072026, 50972020) and the Development of science and technology plan projects of Jilin province (Grant No. 20130206002GX)

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(42) Song,

Y.;

Liu,

G.;

Dong,

X.;

Wang,

J.;

Yu,

W.;

Li,

J.

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of

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Table of Contents Graphic


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Yb3+,Er3+ Multifunctional Hybrid

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