WANG Dan，XUE Bin，TU Lang-ping，ZHANG You-lin，SONG Jun，QU Jun-le，KONG Xiang-gui

（1.Key Laboratory of Optoelectronic Devices and Systems of the Ministry of Education/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:Lanthanide-ion-doped upconversion luminescence is limited by the small absorption cross-section and narrow absorption band of lanthanide ions,which results in weak luminescence.Recently,a dye-sensitized method has proven to be an effective strategy of increasing upconversion luminescence.However,simply attaching dye molecules to nanoparticles with classic Yb-doped nanostructures cannot effectively activate the sensitizing ability of the dye molecules.In response to this problem,we designed Nd-sensitized core/shell/shell(NaYF4:Yb/Er(20/2%)@NaYF4:Yb(10%)@NaYF4:Nd(80%))nanostructures,compared with the classic IR-806 sensitized NaYF4:Yb/Er nanostructure,their upconversion luminescence(500 to 700 nm)was approximately enhanced by a factor of 38.Through analysis of the nanostructure’s emission and luminescence lifetime data,the enhancement was confirmed by the effective overlap of Nd absorption with the emission of near-infrared dye molecules and the protective effects of the shell structure on the luminescent center(the lifetime of Er(4S3/2→4I15/2)was increased by 1.7 times).In addition,we found that the doping Yb3+in the outermost layer will decrease the dye-sensitized luminescence intensity.Furthermore,this Ndsensitized core/shell/shell structure also achieved enhancement in the sensitized upconversion luminescence of the luminescence centers of Ho and Tm,which establishes a foundation for enhanced dye-sensitized upconversion luminescence.

Key words:upconversion luminescence;dye-sensitized;lanthanide ion;nanoparticles

1 Introduction

Due to the unique electron transitions in 4f electron configuration and between 4f and 5d,rare earth ions can generate the photon radiation of various wavelengths from ultraviolet,visible light to infrared light[1-2].The Up-Conversion NanoParticles(UCNPs)doped with rare earth ions have the characteristic of converting two or more low-energy near-infrared photons into a high-energy photon.In particular,the photon emission generated by UCNPs has the advantages such as narrow spectral band,resistance to bleaching,and low background noise[3].This up-conversion luminescence induced by nearinfrared light has been applied in many fields,such as super-resolution imaging,fluorescent labeling,photodynamic therapy,and optical anti-counterfeiting[4-8].

In spite of the above wide applications,the further practical application of the up-conversion luminescence generated by the doped rare earth ions has been limited by its low luminous intensity.How to enhance the up-conversion luminescence has always been an urgent problem to be solved in upconversion luminescence research.In recent years,the techniques such as core-shell nanostructure[9],plasma field-enhanced luminescence[10]and dyesensitized luminescence[11]have realized the enhancement of up-conversion luminescence intensity.In particular,the study of dye-sensitized luminescence has not only enhanced the intensity of up-conversion luminescence,but also broadened the excitation range of up-conversion luminescence.Instead of the rare earth ions with weak absorption(absorption coefficient:0.1～10 M−1cm−1),near-infrared dyes(absorption coefficient:1 000～10 000 M−1cm−1)have been used to absorb near-infrared light so as to realize the up-conversion luminescence enhanced by sensitization.However,the iterated integral of most of the near-infrared dye emission(800～900 nm)and the Yb absorption in the classical Yb-sensitized doping system(950～1,000 nm)is small,thus limiting the enhancement of dye-sensitized up-conversion luminescence.Furthermore,Nd/Er-ion sensitization system has been designed as the recipient of dye sensitization[12-13]to achieve more effective enhancement of dye-sensitized up-conversion luminescence.It is in recent years that Nd3+-sensitized upconversion luminescence system has been developed.Especially,its partitioned doping strategy can realize efficient up-conversion luminescence[14-16].However,the dye-sensitized Nd-ion doping system usually adopts the optimum structural design of Ndsensitized up-conversion luminescence.Considering that the dye will interact with rare earth luminescence system in the dye sensitization process to weaken the luminescence[17],the design and optimization of dye-sensitized Nd-doped up-conversion luminescence system will be more effectively applied in biochemical analysis,tumor diagnosis and treatment,luminescence display and other fields[18-22].

In this paper,Nd-sensitized core/shell/shell structure was designed as the recipient of enhanced dye-sensitized up-conversion luminescence.The Nd-sensitized core/shell/shell structure was successfully prepared by high-temperature thermal decomposition,and then was coupled with the dye IR-806 molecules to enhance the intensity of dye-sensitized up-conversion luminescence.The related structural characterization confirmed the successful preparation of this nanostructure.The enhancement mechanism behind it was further studied through the analysis of emission spectrum and fluorescence lifetime spectrum.Meanwhile,by optimizing the doping concentration of Yb ions in the outermost shell,the intensity of the dye-sensitized up-conversion luminescence without Yb doping proved to be the strongest.

2 Experiment

2.1 Experimental materials

The experimental materials include ytterbium chloride(YbCl3·6H2O,99.99%),yttrium chloride(YCl3·6H2O),erbium chloride(ErCl3·6H2O),sodium hydroxide(NaOH,98%),ammonium fluoride(NHF4,99.99%),IR-780 iodide(99%),4-mercaptobenzoic acid(99%),1-octadecene(ODE),oleylamine(90%)(OM)and oleic acid(90%)(OA),all of which were purchased from Sigma-Aldrich.According to the reference[14],Nd(CF3COO)3was obtained through the reaction between Nd2O3powder and excessive trifluoroacetic acid and then removing the remaining trifluoroacetic acid by evaporation.Ytterbium trifluoroacetate(Yb(CF3COO)3),yttrium trifluoroacetate(Y(CF3COO)3)and sodium trifluoroacetate(CF3COONa)were purchased from GFS Chemicals.Dichloromethane,trichloromethane and dimethyl formamide(DMF)were purchased from Beijing Chemical Works.All the chemical reagents were of analysis purity.

2.2 Synthesis of up-conversion nanoparticles

The core-shell-shell up-conversion nanostructure is fabricated based on the published chemical process[14-15].Firstly,synthesize the core structure.Dissolve YbCl3·6H2O(0.1 mmol),YCl3·6H2O(0.39 mmol)and ErCl3·6H2O(0.01 mmol)in a three-mouth flask containing 3 mL OA and 7.5 mL ODE.Heat the mixture to 150℃for 30 minutes,and then cool it to room temperature under the protection of argon.Then,dissolve NH4F(2 mmol)and NaOH(1.25 mmol)into 5 mL methanol,add the mixture to the above three-mouth flask with rare earth salts,and heat it to 70℃to remove methanol and then heat to 300℃for 1h.Then,add 0.25 mmol NaYF4:Yb(10%)active shell to ODE(synthesized by trifluoroacetate process)and then add to the above mixture for 10 min curing.Then,add 0.5 mmol NaYF4:Nd(20%)active shell(synthesized by trifluoroacetate process)and cure it for 10 min.Finally,cool the solution to room temperature,centrifuge it with ethanol,and dissolve it into 6 mL trichloromethane.The core-shell-shell up-conversion nanostructures doped with different rare earth elements were all synthesized in the similar way.

2.3 Synthesis of IR-780 molecules

Similarly,dissolve organic IR-780 molecules(250 mg),4-mercaptobenzoic acid(115.5 mg)and DMF(10 mL)in a 50-mL three-mouth flask under nitrogen protection according to the Ref.[11].Then keep the mixture under nitrogen protection for 17 h.Filter the product solution with 0.45μm PTFE and remove DMF through reduced-pressure distillation.Then dissolve the residue into 5 mL dichloromethane,filter the mixture again with 0.45μm PTFE and precipitate it with ice ethyl ether.Finally,filter and dry the reactant under vacuum,and keep it in dark place.

2.4 Synthesis of IR-806 molecules

In the similar way as Refs.[8],[17],dissolve 1 mL IR-806(xmg/mL,x=0～20 mg/mL)into trichloromethane,and mix it with 1 mLβ-NaYF4:Yb/Er(20/2%)@NaYF4:Yb(10%)@NaYF4:Nd(20%)nanoparticles(Er3+:～1.67 mmol).Stir the entire reaction mixture for 24 h at room temperature,centrifuge it,and redisperse it into 1 mL trichloromethane.Test the up-conversion spectrum of IR 806-sensitized UCNPs under the Er3+concentration of about 16μmol.

2.5 Experimental characterization testing

Transmission Electron Microscope(TEM)was tested at 200 kV by use of Tecnai G2 F20 S-Twin electron microscopy.X-ray diffraction(XRD)test was performed on Rigaku D/Max-2000 by using Cu Kαradiation(λ=0.154 1 nm)as diffraction radius.Absorption spectrum was tested on a Maya 2000 spectrometer(Ocean Optics).The up-conversion spectrum was recorded by an externally coupled 808 nm laser on an ocean optical spectrometer(Maya2000).Energy Dispersive Spectrum(EDS)analysis was characterized by Hitachi S-4800.In the fluorescence lifetime test of up-conversion luminescence,500 MHz TDS 3052 was used as the excitation light source,and the fluorescence lifetime data was obtained through an OPO(Sunlite 8000)and an oscilloscope.

3 Experimental results and discussion

3.1 Morphology and characterization of nanoparticles

The highly uniform Nd3+-sensitized core-shellshell(NaYF4:Yb/Er(20/2%)@NaYF4:Yb(10%)@NaYF4:Nd(80%))UCNP structure was successfully prepared through high-temperature thermal decomposition.The TEM photo showed that the UCNPs were in uniform size.As shown in Fig.1,the average sizes of core(NaYF4:Yb/Er(20/2%),denoted as“C”),core/shell(NaYF4:Yb/Er(20/2%)@NaYF4:Yb(10%),denoted as“CS”)and core/shell/shell(NaYF4:Yb/Er(20/2%)@NaYF4:Yb(10%)@NaYF4:Nd(80%)),denoted as“ CSS”)were 23.5 nm,26.3 nm and 33.6 nm,respectively.This increasing size confirmed that an Yb transition layer of about 1.4 nm and a 3.7 nm Nd-sensitized nanoshell layer were gradually growing on the NaYF4:Yb/Er nano-core.The Fig.2(a)shows that the UCNPs have a classical hexagonal phase structure(JCPDS-16-0334).The EDS confirmed that Nd,Y,Yb and other rare earth elements were effectively doped into the nanoparticles(Fig.2(b)).Furthermore,we synthesized the IR-806 molecule according to the reference method(Fig.2(c)).As seen from the absorption diagram(Fig.2(d)),its absorption peak shifted from 780 nm to 806 nm,which confirmed the successful synthesis of IR-806 molecule.Furthermore,the IR-806 molecules was modified to the surface of UCNPs according to the above method.As shown in Fig.2(d),after an IR-806 molecule was modified to UCNPs,the absorption peak of the UCNPs was masked by the absorption spectrum of IR-806,thus confirming the successful modification of dye molecule to UCNPs.

Fig.1 TEM images of core(a),core/shell(b)and core/shell/shell(c)of up-conversion nanoparticles and their size distributions(d)图1 上转换纳米粒子的核（a），核/壳（b），核/壳/壳（c）电镜表征图及尺寸分布（d）

Fig.2 (a)XRD data of C,CS and CSS of UCNPs and theβ-NaYF4(JCPDS-16-0334,bottom);(b)EDS data of CSS;(c)IR-806 synthesis process;(d)absorptions of IR-780 and IR-806 before and after synthesis;(e)absorption of UCNPs and dye conjugated UCNPs after IR-806 connection图2 （a）上转换纳米粒子的核、核/壳、核/壳/壳XRD 及标准卡片β-NaYF4(JCPDS-16-0334,底部)结果，（b）上转换CSS 的EDS 数据，（c）IR-806合成过程图，（d）合成前后IR-780和IR-806 的吸收，（e）连接IR-806 之后UCNPs 吸收和UCNPs 本身的吸收

3.2 Confirmation and discussion of dye-sensitized enhancement mechanism

As shown in Fig.3(a),the up-conversion luminescence intensity of the dye-sensitized CSS structure proposed in this paper is about 38 times stronger than that of the classical IR-806-sensitized NaYF4:Yb/Er(20/2%)nanoparticle reported at the earliest[11].This proves that the dye-sensitized structure has achieved the enhancement of up-conversion luminescence intensity.In addition,under the excitation of 808 nm near-infrared light,the intensities of both the up-conversion red and green light emissions of dye-sensitized CSS structure were nonlinearly dependent on excitation light power(Fig.3(b)).The corresponding multiphoton indexes were 1.67(540 nm green light emission:4S13/2→4I15/2)and 2.0(655 nm red light emission:4F9/2→4I15/2).Therefore,the luminescence of this structure proves to be nonlinear up-conversion luminescence.

Fig.3 (a)Up-conversion luminescence(UCL)spectra of IR-806-sensitized CSS structure and IR-806-sensitized core structure under 808 nm excitation wavelength;(b)log-log plots of the UCL intensity versus laser power for the IR-806 dye-sensitized CSS under 808 nm excitation图3 （a）IR-806 敏化CSS 结构的上转换光谱及IR-806 敏化核结构的上转换光谱，激发波长为808 nm，（b）808 nm 激发下的IR-806 敏化的CSS 结构的上转换发光强度随功率变化的log-log 关系

According to our analysis,the outermost layer of CSS nanostructure is Nd3+-doped shell,where Nd can be sensitized by IR-806 molecules efficiently due to the serious overlap between the Nd absorption and the emission of IR-806 dye molecules,as shown in Fig.4(a)(Color online).On the other hand,the nanoshell in CSS structure can effectively protect the luminescence center.As seen from Fig.4(b)(Color online),the luminescence lifetime of Er(253μs)in the dye-sensitized CSS structure was significantly longer than that of Er in the dyesensitized core nanoparticle(146μs)or core nanoparticle(169μs).In other words,the luminescence lifetime of Er has increased by 1.73 times and 1.50 times,respectively.Thus,it is proved that the nanoshell can insulate the luminescence center from the external interference environment so as to guarantee a long life of the luminescence center.It is worth noting that although the designed dye-sensitized Nd3+-doped nanostructure has a better excitation wavelength near 800 nm,but NaYF4:Yb/Er(20/2%)nanoparticles can only be excited by 980 nm wavelength.As shown in Fig.4(b),the exicitation wavelength was 980 nm,not the conventional 808 nm wavelengh.Furthermore,the Fig.4(c)shows that the luminescence life of CSS nanostructure remains unchanged whether dye molecules are connected or not.This further proves that the nanoshell can effectively prevent the interaction between the luminescence center and the external environment,thus enhancing the up-conversion luminescence

The difference of Nd3+sensitization system lies in the fact that its outermost nanoshell is doped with only Nd3+ions,rather than the previously reported Nd-Yb ions[16].This structural design is based on the results of our experiments.As shown in Fig.5,with the increase of Yb3+-doping concentration in the outermost layer,the up-conversion luminescence intensity of dye-sensitized CSS structure will gradually decrease.According to our previous research of dye-sensitized rare earth up-conversion nanosystem,the excitation energy absorbed by dye needs to be gradually transferred to the internal luminescence center[17].In this process,the energy loss in the migration of excitation energy to the surface is very heavy.The doped Yb3+ions are likely to transfer the excitation energy to the surface[9,23-24],thus reducing the excitation energy transferred to the interior and weakening the up-conversion luminescence.Therefore,the strongest dye-sensitized up-conversion luminescence is produced in the outermost nanoshell without doped Yb3+ions,as shown in Fig.5.

Fig.4 (a)Overlap between the emission spectrum of IR-806 molecules and the absorption spectrum of Nd3+;(b)the lifetimes of Er3+(4S3/2→4I15/2)in core nanoparticles(black,NaYF4:Yb/Er(20/2%),dye-sensitized core nanoparticles(red)and dye-sensitized CSS nanostructure(blue)under 980 nm excitation;(c)the lifetimes of Er3+(4S3/2→4I15/2)in CSS nanostructure and dye-sensitized CSS nanostructure under 808 nm excitation图4 （a）IR-806 分子的发射光谱与Nd3+的吸收交叠图；（b）980 nm 激发下，测试得到的核纳米粒子（黑色,NaYF4:Yb/Er(20/2%)），染料敏化核纳米粒子（红色）及染料敏化的CSS 纳米结构（蓝色）的Er3+(4S3/2→4I15/2)的寿命测试结果；（c）808 nm 激发下CSS 纳米结构及染料敏化的CSS 纳米结构的Er3+(4S3/2→4I15/2)的寿命测试结果

Fig.5 Upconversion spectra of dye-sensitized NaYF4:Yb/Er(20/2%)@NaYF4:Yb(10%)@NaYF4:Nd/Yb(80/x%)(x=0,5,10,20)nanoparticles under 808 nm excitation图5 染料敏化的NaYF4:Yb/Er(20/2%)@NaYF4:Yb(10%)@NaYF4:Nd/Yb(80/x%)(x=0,5,10,20)的上转换光谱（808 nm 激发）

3.3 Enhancement of dye-sensitized luminescence by using Ho or Tm as luminescence center

Furthermore,the replacement of luminescence center in the core of CSS structure by Ho(NaYF4:Yb/Ho(20/1%))@NaYF4:Yb(10%)@NaYF4:Nd(80%))or Tm(NaYF4:Yb/Tm(20/1%)@NaYF4:Yb(10%)@NaYF4:Nd(80%))also realized the enhancement of dye-sensitized up-conversion luminescence(Fig.6(a)and 6(b),Color online).When the luminescence center was Ho or Tm,the typical nonlinear dependence of luminescence intensity on excitation light power was also shown(Fig.6(c)and 6(d),Color online).When the luminescence center was Ho,the multi-photon indexes were 1.57(540 nm emission,4S13/2→4I15/2)and 1.88(645 nm emission,4F9/2→4I15/2)respectively.When the luminescence center was Tm,the multi-photon indexes were 2.82(450 nm emission,1D2→3F4),1.74(470 nm emission,1G4→3H6),1.80(645 nm emission,1G4→3F4)and 1.34(695 nm emission,3F2→3H6)respectively.It should be noted that,for Tm ions,the dye-sensitized up-conversion luminescence had hardly been seen in the NaYF4:Yb/Tm(20/1%)nano-core structure.This is because the 800 nm emission level(3H4→3H6)of Tm heavily overlapped with the absorption of IR-806 molecules,thus quenching the Tm luminescence.However,the outer shell of CSS structure successfully blocked the transfer of Tm to IR-806,thus realizing the dye-sensitized up-conversion luminescence.

Fig.6 (a)The UCL of the IR-806 sensitized Ho core nanostructure and IR-806 sensitized Ho-CSS nanostructure,(b)the UCL of the IR-806 sensitized Tm core nanostructure and IR-806 sensitized Tm-CSS nanostructure,(c)log-log plots of the UCL intensity over laser power for the green and red emissions of the dye-sensitized Ho-CSS under 808 nm excitation,(d)log-log plots of the UCL intensity versus laser power for the green and red emissions of the dye-sensitized Tm-CSS under 808 nm excitation图6 （a）IR-806 敏化Ho 核结构及IR-806 敏化Ho-CSS 结构的上转换光谱，（b）IR-806 敏化Tm 核结构及IR-806 敏化Tm-CSS 结构的上转换光谱，（c）808 nm 激发下的IR-806 敏化的Ho-CSS 结构的上转换发光强度随功率变化的loglog 关系，（d）808 nm 激发下的IR-806 敏化的Tm-CSS 结构的上转换发光强度随功率变化的log-log 关系

4 Conclusion

Highly uniform NaYF4:Yb/Er(20/2%)@NaYF4:Yb(10%)@NaYF4:Nd(80%)up-conversion nanoparticles were successfully prepared.Their up-conversion luminescence intensity was about 38 times stronger than that of dye-sensitized NaYF4:Yb/Er(20/2%)core nanostructure.Further studies showed that there were two reasons for this enhancement.On the one hand,the heavy overlap between the Nd absorption in the outmost layer and the emission of dye IR-806 molecules led to the effective absorption of the excitation energy of dye.On the other hand,due to the protective effect of nanoshell layer on the luminescence center,the luminescence life of this structure was 1.73 times longer than that of dye-sensitized core nanostructure.By changing the doping concentration of Yb3+ions in the outermost layer,we demonstrated that the dye-sensitized up-conversion luminescence would be weakened by the doping of Yb3+ions,and could become the strongest without the doping of Yb3+ions.Furthermore,this dye-sensitized CSS structure has realized the enhancement of dye-sensitized up-conversion luminescence intensity when the luminescence center is Ho or Tm.

——中文对照版——

1 引言

稀土离子由于其具有4f 电子组态内及4f 到5d 之间的电子跃迁的特异性，导致其可产生从紫外、可见光区到红外光区的多种波长的光子辐射[1-2]。尤其是，稀土离子掺杂的上转换纳米粒子(UCNPs)具备将两个及两个以上的低能量近红外光子转换为一个高能量光子的特性，其产生的光子发射具有谱带窄、抗漂白、背景噪声低等优点[3]。这种近红外光激发的上转换发光特性将产生诸多应用，如超分辨成像、荧光标记、光动力治疗、光学防伪等[4-8]。

尽管稀土离子掺杂产生的上转换发光有诸多应用，但其相对较低的发光强度限制了其进一步实际应用。……