Enhancing Single-band Red Upconversion Luminescence of Ho3 + Through Dye-sensitization

WANG Dan XUE Bin TU Lang-ping ZHANG You-linSONG Jun* QU Jun-le KONG Xiang-gui*

(1. Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province,College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China；2. College of Information Engineering, Shenzhen University, Shenzhen 518060, China；3. State Key Laboratory of Luminescence and Applications, Changchun Institute of Optics, Fine Mechanics and Physics,Chinese Academy of Sciences, Changchun 130033, China)

Abstract： Single-band red upconversion emission has important applications for high-resolution biomarkers and 3-dimensional color display. However, the cross-relaxation of Ho3+ /Ce3+ co-doping system normally yield the weak single-band red emission. In this paper, the dye-sensitized NaYF4 ∶Yb/Ho/Ce(20% /2% /10%)@ NaYF4 ∶Nd(20%) nanoparticles were prepared to address such an issue. Highly uniform upconversion nanoparticles were prepared by solvothermal method, and the single-band red emission was obtained by increasing the Ce3+ doping concentration (from 0% to 10%) inside the core. When the near-infrared dyes of IR-806 were further conjugated, the upconversion luminescence intensity exhibited a 22-fold enhancement,and the red-to-green emission ratio also increased from 4.8 to 8.4. This paper provides a new way to improve the intensity and the purity of single-band red upconversion luminescence through dye sensitization, which is beneficial to high-resolution bioimaging.

Key words： upconversion luminescence； single-band red emission； dye-sensitization； nanoparticles

1 Introduction

Lanthanide ions(Ln3+), characterized by unique 4fnelectronic configuration, can generate the radiative emission ranging from the ultraviolet to the infrared region[1-2]. Especially,Ln3+-doped upconversion nanoparticles(UCNPs) can convert two or more low-energy near-infrared photons into high-energy ultraviolet/visible photons. Their features of the large anti-Stokes shift, narrow emission band, long luminescence lifetime[3-4], could find promising applications in bioimaging, photodynamic therapy and display devices[5-8]. Moreover, due to that biological tissues have weak light scattering and absorption fornear-infrared(NIR) light, NIR-excited upconversion luminescence is of particular interest in biomedicine[9-10].

As well known, single-band red emission has excellent applications in 3D full-color display and high-resolution bioimaging[11]. At present, to achieve the bright single-band red emission, Yb/Er codoping systems were most widely studied by further doping with Mn2+or Tm3+ions. In such systems, doping ions could induce the cross-relaxation process with Er3+, leading to the quenching of the green emission and consequent increase of the red to green(R/G) emission ratio[12-14]. Similarly, introducing Ce3+ions into the Yb/Ho upconversion luminescence system could also cause the cross-relaxation process and increase the R/G luminescence intensity ratio[15]. However, the obtained single-band red upconversion emission is normally weak, which is unfavorable for the following applications such as high resolution of biological imaging. Although the cross-relaxation strategy could produce the single band red emission, the total upconversion luminescence(UCL) was greatly reduced. Therefore, how to produce single-band red emission while simultaneously maintaining the UCL intensity becomes a critical issue need to be addressed.

In this paper, we reported the preparation of the dye-sensitized NaYF4∶Yb/Ho/Ce(20% /2% /10%)@NaYF4∶Nd(20%) nanoparticles towards enhanced single-band red upconversion luminescence of Ho3+. The UCNPs were first prepared through a facile solvothermal method,and then conjugated with the dye molecules of IR-806. The UCL was studied by varying the doping concentration of Ce3+.

2 Experiments

2.1 Synthesis of UCNPs

NaYF4∶Yb/Ho/Ce(20% /2% /x%,x=0, 3,5,7,10)@NaYF4∶Nd(20%) core/shell nanoparticles were synthesized according to previous reports[16-17]. For synthesizing 0.5 mmol samples,YCl3·6H2O(99.9%,0.395,0.38, 0.37, 0.36, 0.345 mmol), CeCl3·6H2O(99.9%,0,0.015,0.025,0.035,0.05 mmol), YbCl3·6H2O(99.9%, 0.1 mmol) and HoCl3·6H2O(99.9%, 0.005 mmol)were dispersed in a solution containing 3 mL of OA and 7.5 mL of ODE. The suspension was first heated at 155 ℃for 0.5 h to get a clearLn-OA complex solution. Then NH4F(2 mmol) and NaOH(1. 25 mmol) dissolved in 6 mL of methanol were added to the reaction solution. Afterwards, the solution was heated to 70 ℃to remove the methanol and subsequently heated at 300 ℃for 1 h. After that,1 mmol of NaYF4∶Nd(20%) active shell solution was injected into the above reaction mixture and maintained 15 min for ripening.Finally, the solution was cooled down to room temperature and precipitated using ethanol for three times.

2.2 Synthesis of IR-806 Modified UCNPs

According to the reported methods[18-19], the IR-780 dye molecule was first carboxylated to obtain the IR-806 dye. Dimethylformamide(10 mL), IR-780 molecule(250 mg), 4-mercaptobenzoic acid(115.5 mg) were mixed and dissolved. After reaction for 17 h under argon, the solution was filtered and distilled under reduced pressure to remove dimethylformamide. The obtained product was re-dissolved in 5 mL of dichloromethane, followed by further filtration, precipitation with glacial ether and vacuum drying. The resulting product was IR-806 and was kept in the dark for use. For surface conjugation of UCNPs with dyes, the obtained 1 mL IR-806(2 mg/mL) and 10 mg NaYF4∶Yb/Ho/Ce(20%/2%/10%)@NaYF4∶Nd(20%) nanoparticles(dissolve in chloroform),were mixed and stirred at room temperature under dark conditions for 24 h, then centrifuged and re-dispersed in 1 mL of chloroform.Finally, the surface-modified UCNPs of IR-806 were diluted 200-fold for further spectroscopic analysis.

2.3 Characterizations

The power X-ray diffraction(XRD) patterns of the UCNPs were recorded on a Rigaku Miniflex 600 bench top powder X-ray diffraction instrument with Cu-Kα (λ=0.154 nm) radiation, operating at 40 kV and 15 mA. The morphology and particle size distributions of the samples were also checked using a FEI Tecnai G220 transmission electron microscope(TEM). Fourier transform infrared (FTIR) spectra were recorded with 2 cm-1resolution (Paragon 1000, Perkin-Elmer, USA). The room-temperature photoluminescence emission spectra were measured on a Horiba Fluoromax-3 fluorometer upon excitation with a continuous 808 nm fiber coupled diode lasers. The absorption spectra were acquired on a Maya 2000 spectrometer(Ocean Optics).

3 Results and Discussion

3.1 Characterizations of Nanoparticles

As shown in Fig. 1, transmission electron microscopy showed that UCNPs were uniform in size and the particle size was approximately 31 nm. XRD data showed(Fig.2(a)) that the diffraction peaks of the prepared nanoparticles were in agreement with the standard NaYF4hexagonal phase pattern(JCPDS：16-0334), confirmed that the prepared nanoparticles were hexagonal phase structure. Next, the IR-806 dye was prepared by carboxylation of a commercially available IR-780 dye(Fig. 2(b)). As shown in Fig.2(c), the absorption peak of the dye molecule was centered at 806 nm, confirming that the IR-806 dye was successfully prepared. On the other hand, the infrared absorption spectra analysis of the structure of dye-sensitized UCNPs(UCNPs@dye) also confirmed the success of conjugating dyes on the surface of UCNPs. Fig.2(d) shows the FTIR spectra of the obtained UCNPs and UCNPs@dye.Due to the presence of oleic acid ligands on the surface of UCNPs, infrared vibration peaks of 2 929 and 2 850 cm-1were observed, which correspond to the symmetry and antisymmetric vibrations of the methylene group of the oleic acid ligands, respectively. After further conjugation with the IR-806 molecules, in addition to the vibration modes of oleic acid molecules, a new vibration peak appeared at 1 251 cm-1for UCNPs@dye,which corresponded to the C-O stretching mode of IR-806. The appearance of this new vibration peak suggested that IR-806 molecules were coordinated on the surface of UCNPsviacarboxyl group. Moreover, a broad absorption peak( ～806 nm) appeared well following that of dye conjugation, which further indicated the successful synthesis of UCNPs@dye.

Fig.1 (a)TEM image of NaYF4 ∶Yb/Ho/Ce(20% /2% /10%)@NaYF4∶Nd(20%) upconversion nanoparticles. (b)Size distribution of UCNPs.

Fig.2 (a)XRD patterns of UCNPs and the standard card of β-NaYF4(JCPDS-16-0334). (b)Synthesis process of IR-806. (c)Absorption spectrum of IR-806. (d)FTIR spectra of UCNPs and UCNPs@dye. (e)Absorption spectra of UCNPs and dye-sensitized UCNPs.

3.2 Upconversion Luminescence of Ho3+ in NaYF4∶Yb/Ho/Ce@NaYF4∶Nd Nanopartices

The designed NaYF4∶Yb/Ho/Ce@ NaYF4∶Nd(20%) nanostructure could produce the single-band upconversion red emission upon 808 nm excitation due to the cross-relaxation(CR) strategy between Ho3+and Ce3+. According to previous reports[20-21],the Nd3+ions sensitized core-shell nanostructure was adopt to avoid the quenching effects between the activators and Nd3+ions. The Nd3+-shell could suppress the surface-related quenching and further transfer excitation energy to the activators located in the inner core for UCL. Fig.3(a) shows the luminescence spectra of NaYF4∶Yb/Ho/Ce(20% /2% /x%,x=0,3,5,7,10)@NaYF4∶Nd(20%) UCNPs. The green emission(540 nm) and the red emission(645 nm) originating from the5S2,5F4→5I8and5F5→5I8transitions of Ho3+respectively, were observed. As shown in Fig.3(e), the luminescence mechanism follows the first energy migration from Nd3+to Ho3+viaYb3+,and subsequent energy transfer upconversion(ETU) with5I6as the metastable level. Fig.3(b) shows that as the Ce3+doping concentration increased, the R/G emission ratio gradually increased. In addition, as shown in Fig.3(c),the overall upconversion luminescence intensity gradually decreased with increasing Ce3+doping concentration. Moreover, as shown in Fig. 3(d),the Commission Internationale de L'Eclairage(CIE)chromaticity coordinate(calculated using the colorimetry system as suggested in the CIE 1931) of the upconversion luminescence of different samples also indicated that the luminescence gradually moved from the green to the red region. According to previous report[15], the observed luminescence evolution with Ce3+doping could be reasonably explained as being due to the three CR processes (Fig.3(e))：CR1：5I6(Ho3+) +2F5/2(Ce3+)→5I7(Ho3+) +2F7/2(Ce3+),CR2：5S2/5F4(Ho3+) +2F5/2(Ce3+)→5F5(Ho3+) +2F7/2(Ce3+), CR3：5F5(Ho3+) +2F5/2(Ce3+)→5I4(Ho3+) +2F7/2(Ce3+). The CR process induced by Ce3+was more effective to quench the green emission of Ho3+than red emission, resulting in a significant increase in the R/G emission ratio (Fig.3(b)). However, the effective CR process also greatly decreased the overall UCL intensity, as shown in Fig.3(c), more than 90% of UC emission was quenched when doping 10%Ce3+.Therefore, how to maintain the intensity of singleband red upconversion luminescence becomes an issue needed to be addressed.

Fig.3 (a)UC luminescence spectra of the samples of NaYF4 ∶Yb/Ho/Ce(20% /1% /x%, x =0,3,5,7,10)@ NaYF4 ∶Nd(20%) UCNPs under 808 nm excitation. The increased R/G(b) and the decreased luminescence intensity(c) with the doping concentration of Ce3+. (d)CIE chromaticity diagram of the samples with different Ce3+ concentrations (a-e corresponding to x=0,3,5,7,10). (e)Schematic diagram of the energy levels and the proposed UC mechanism.

3. 3 Upconversion Luminescence of NaYF4 ∶Yb/Ho/Ce@NaYF4∶Nd@dye

To achieve the enhancement of single-band red upconversion luminescence, we designed and prepared the dye-sensitized NaYF4∶Yb/Ho/Ce(20% /2% /10%)@NaYF4∶Nd(20%) UCNPs, where the Nd3+ions located in outer layer served as energetic acceptors from IR-806 before further transferring the excitation energy to the internal luminescence centers,due to the fact that Nd3+ions have multiple absorption bands overlapping with the emission of the near-infrared dyes of IR-806[19-20]. As shown in Fig.4(a), the single-band red upconversion luminescence of UCNPs was enhanced by a factor of 22.Similar to our previous report[19],this significant upconversion luminescence enhancement was mainly attributed to the strong near-infrared light absorption ability of the conjugated dye molecules. The dyes could transfer a large amount of absorbed excitation energy to Nd3+, and then further to the internal luminescent centers, as a consequence, the upconversion luminescence was enhanced greatly. It should be noted that, as shown in Fig.4(b), the dye-sensitization also increased the R/G emission ratio of Ho3+from 4. 9 to 8. 4. According to our previous publication[22], such results could be explained as bellow. When the slope of the power-dependent multi-photon processes of the red emission was larger than that of green emission, the R/G emission ratio increased with excitation power density. Herein,dye sensitization played a role of increasing excitation power due to their larger absorption cross-section at the excitation wavelength, then it could increase the R/G emission ratio under the equivalent excitation power density. We tested the power-dependent multi-photon processes of the NaYF4∶Yb/Ho/Ce(20% /2% /10%)@ NaYF4∶Nd(20%) UCNPs.Fig.4(c) shows the double logarithm plot resulting from the luminescence intensityversuspump power density, where the slope values ofnfor the5S2,5F4→5I8(540 nm) and5F5→5I8(645 nm) transitions of the Ho3+ions were 2. 0, 1. 9, respectively. The slope of the red emission was larger than that of green emission. The same case holds for the dyesensitized NaYF4∶Yb/Ho/Ce(20% /2% /10%)@NaYF4∶Nd(20%) UCNPs(Fig.4(d)). It is obvious that the intensity of the red emission increases faster than the green emission with the excitation power density. Therefore, the R/G will gradually increase with the excitation power density. The effect of dye-sensitization is similar to increase the excitation power[22], and could therefore increase the R/G ratio. Therefore, dye-sensitization not only increased the intensity of single-band red upconversion luminescence of Ho3+, but also improved the red purity of the UC emission.

Fig.4 (a)UC luminescence spectra of NaYF4 ∶Yb/Ho/Ce(20% /2% /10%)@ NaYF4 ∶Nd(20%) UCNPs(black line) and dye-sensitized UCNPs(red line). (b)Increased R/G ratio after conjugating UCNPs with dyes. Lg-lg plots of the UCL intensity versus laser power density for the green and red emissions of the UCNPs(c) and dye-sensitized UCNPs(d) under excitation of 808 nm.

4 Conclusion

In this work, highly uniform hexagonal phase NaYF4∶Yb/Ho/Ce(20% /2% /10%)@NaYF4∶Nd(20%) UCNPs were prepared by solvothermal method, and thus IR-806 molecules were successfully conjugated on the surface of UCNPs. By varying the Ce3+doping concentration(0% ～10%),the R/G of Ho3+gradually increased with the Ce3+doping concentration. Especially for 10%Ce3+doping concentration, a single-band red UC luminescence was achieved, though more than 90% of the total UCL intensity was quenched by Ce3+ions. After further surface conjugation with near-infrared IR-806 molecules,the luminescence intensity of the UCNPs was significantly increased by a factor of 22. More importantly,the R/G emission ratio of Ho3+increased from ～4.9 to 8.4. Therefore,the dye-sensitization method not only enhanced Ho3+single-band upconversion luminescence intensity, but also improved the red emission purity,which is beneficial to single-band red emission for high-resolution bioimaging.