1.

Introduction

In recent years, water-soluble fluorescent nanoparticles have been applied often in biological label applications.^1,2 Among the fluorescent nanoparticles, rare-earth ion–doped nanoparticles have played an important role. They show superior chemical and optical properties, including low toxicity, large effective Stokes shifts, sharp emission band widths of 10–20 nm, and high resistance to photobleaching, blinking, and photochemical degradation.³ The problem with rare-earth ion-doped nanoparticles is their weak luminescence efficiency. Consequently, a great deal of research work has been devoted to improving luminescence efficiency of rare-earth ion–doped nanoparticles.^4,5

${\mathrm{NdF}}_3$ nanoparticles have been reported to lack the concentration quenching effect and have very low phonon energy.⁶ ${\mathrm{NdF}}_3$ nanoparticles coated with silica shells have been used in biological detection for near-infrared (NIR) luminescence.⁷ When ${\mathrm{NdF}}_3$ nanoparticles disperse in water, the luminescence intensity is weakened. Improving the photoluminescence (PL) of water-soluble ${\mathrm{NdF}}_3$ nanoparticles is urgent.

It is possible to tune the emission intensity of rare-earth ions by modifying neighboring network environment by introducing other atoms into the host lattice. Commonly, incorporation of metal atoms into host lattice can distort the lattice and modify the energy absorption and transfer behaviors, resulting in increased emission intensity of rare-earth ions. The reason is that the doped metal atoms distort the local symmetry around rare-earth ions and modify the selection rules of rare earth ions.^8–10

In this study, water-soluble ${\mathrm{NdF}}_3$, ${\mathrm{NdF}}_3:{\mathrm{Li}}^{+}$, and ${\mathrm{NdF}}_3:{\mathrm{Ba}}^{2+}$ nanoparticles coated with polyvinylpyrrolidone (PVP) were synthesized by a facile hydrothermal method. Morphology and luminescence properties were studied. In comparison with ${\mathrm{NdF}}_3$ nanoparticles, codoping with ${\mathrm{Ba}}^{2+}$ ions improves the NIR emissions of ${\mathrm{NdF}}_3$, whereas codoping with ${\mathrm{Li}}^{+}$ ions does not appreciably change the emission intensity. The reason for the phenomena is discussed below.

2.

Methods

${\mathrm{NdF}}_3$ and ${\mathrm{NdF}}_3/\mathrm{PVP}$ nanoparticles were prepared via a hydrothermal method previously described.⁹ The ${\mathrm{NdF}}_3:{\mathrm{Li}}^{+}/\mathrm{PVP}$ nanoparticles were prepared by adding 0.3 g PVP and 5% ${\mathrm{LiNO}}_3$ into the solution, and ${\mathrm{NdF}}_3:{\mathrm{Ba}}^{2+}/\mathrm{PVP}$ nanoparticles were prepared by adding 0.3 g PVP and 5% ${\mathrm{BaCl}}_2$ into the solution.

The powder x-ray diffraction (XRD) patterns were tested by Philips PW1830 diffractometer using $\mathrm{Cu}\mathrm{K}\alpha$ irradiation at 40 kV and 40 mA. The morphologies were imaged with field-emission scanning electron microscopy (FE-SEM) (Nova NanoSEM 430) operating at 10 Kv. The PL spectra were measured at room temperature through a spectrometer (Jobin-Yvon Triax320) with an 808-nm laser diode as excitation source recorded with a liquid nitrogen–cooled InGaAs detector.

3.

Results

Figure 1 shows the typical XRD patterns of the synthesized nanoparticles. All the diffraction peaks are well matched with theoretical data for hexagonal-phase structure of ${\mathrm{NdF}}_3$, with lattice constants $a=0.7030\mathrm{nm}$ and $c=0.7199\mathrm{nm}$ (Joint Committee on Powder Diffraction Standards no. 9-416). No other impurity peaks were detected. These results indicate that the coating of PVP does not change the hexagonal phase of ${\mathrm{NdF}}_3$ nanoparticles and the codoping ${\mathrm{Li}}^{+}$ and ${\mathrm{Ba}}^{2+}$ ions completely entered the host lattice without formation of any additional phase at codoped concentrations.

Fig. 1

XRD patterns of ${\mathrm{NdF}}_3$, ${\mathrm{NdF}}_3/\mathrm{PVP}$, ${\mathrm{NdF}}_3:{\mathrm{Li}}^{+}/\mathrm{PVP}$, and ${\mathrm{NdF}}_3:{\mathrm{Ba}}^{2+}/\mathrm{PVP}$ nanoparticles.

The FE-SEM images of ${\mathrm{NdF}}_3$, ${\mathrm{NdF}}_3/\mathrm{PVP}$, ${\mathrm{NdF}}_3:{\mathrm{Li}}^{+}/\mathrm{PVP}$, and ${\mathrm{NdF}}_3:{\mathrm{Ba}}^{2+}/\mathrm{PVP}$ nanoparticles are shown in Fig. 2. The ${\mathrm{NdF}}_3$ particles are monodisperse nanoplates, as shown in Fig. 2(a). The diameter of the plates is estimated to be in the range of 80–120 nm, whereas the thickness is about 30 nm. When coating with water-soluble PVP, the morphology of the nanoplates is nearly the same as that of the former particles, as shown in Fig. 2(b). Then codoped with ${\mathrm{Li}}^{+}$ ions, as shown in Fig. 2(c), the morphology does not have any distinct change compared to Fig. 2(b). When codoped with ${\mathrm{Ba}}^{2+}$, the morphology has changed, as shown in Fig. 2(d): the diameter of the nanoparticles became more uniform and decreased to about 60 nm, while the thickness has not changed, indicating that ${\mathrm{Ba}}^{2+}$ ions inhibit the growing of nanoplates in the direction of the plane. The smaller size of ${\mathrm{NdF}}_3:{\mathrm{Ba}}^{2+}/\mathrm{PVP}$ nanoparticles is of benefit for the use of biological labels.

Fig. 2

FE-SEM images of ${\mathrm{NdF}}_3$ (a), ${\mathrm{NdF}}_3/\mathrm{PVP}$ (b), ${\mathrm{NdF}}_3:{\mathrm{Li}}^{+}/\mathrm{PVP}$ (c), and ${\mathrm{NdF}}_3:{\mathrm{Ba}}^{2+}/\mathrm{PVP}$ (d) nanoparticles.

The PL spectra of the nanoparticles under 808-nm laser excitation at room temperature are shown in Fig. 3. The spectra represent the main transition of ${\mathrm{Nd}}^{3+}$ ions at 1060 nm corresponding to ${}^4{F}_{3/2}\to {}^4{I}_{11/2}$ channel. The coating of PVP layer increases the PL intensity of ${\mathrm{NdF}}_3$ at 1060 nm by 16%. The increase is perhaps due to the PVP layer, which prevents the energy losses from surface quenching of ${\mathrm{NdF}}_3$ nanoparticles.¹¹

Fig. 3

Fluorescence spectra of ${\mathrm{NdF}}_3$, ${\mathrm{NdF}}_3/\mathrm{PVP}$, ${\mathrm{NdF}}_3:{\mathrm{Li}}^{+}/\mathrm{PVP}$, and ${\mathrm{NdF}}_3:{\mathrm{Ba}}^{2+}/\mathrm{PVP}$ nanoparticles under 808-nm laser excitation.

4.

Discussion

When codoping with ${\mathrm{Li}}^{+}$ ions into ${\mathrm{NdF}}_3/\mathrm{PVP}$ nanoparticles, the NIR emission intensity does not have any appreciable change, unlike the PL enhancement we reported in ${\mathrm{Y}}_2{\mathrm{O}}_3$ nanocrystals codoped with ${\mathrm{Li}}^{+}$ ions. But when divalent ions ${\mathrm{Ba}}^{2+}$ are codoped into ${\mathrm{NdF}}_3/\mathrm{PVP}$ nanoparticles, the NIR emission intensity increases further by 25% at 1060 nm.

Codoping of ${\mathrm{Li}}^{+}$ ions (5%) did not change the emission intensity of ${\mathrm{NdF}}_3/\mathrm{PVP}$ nanoparticles, whereas codoping of ${\mathrm{Ba}}^{2+}$ ions (5%) enhanced the NIR emission. The results could be due to the different ionic radius. The ionic radius for ${\mathrm{Li}}^{+}$, ${\mathrm{Ba}}^{2+}$, and ${\mathrm{Nd}}^{3+}$ ions are 76, 135, and 98.3 pm, respectively. ${\mathrm{Ba}}^{2+}$ ions have large ionic radii, which might distort the lattice network of ${\mathrm{Nd}}^{3+}$ ions in ${\mathrm{NdF}}_3$, resulting in the enhanced PL intensity. The smaller ${\mathrm{Li}}^{+}$ ions might be unable to distort the lattice network of ${\mathrm{Nd}}^{3+}$ ions in ${\mathrm{NdF}}_3$ enough to influence the PL intensity.

5.

Conclusions

We have presented a simple synthesis method of water-soluble ${\mathrm{NdF}}_3/\mathrm{PVP}$, ${\mathrm{NdF}}_3:{\mathrm{Li}}^{+}/\mathrm{PVP}$, and ${\mathrm{NdF}}_3:{\mathrm{Ba}}^{2+}/\mathrm{PVP}$ nanoparticles by coating with a layer of hydrophilic polymer PVP via hydrothermal method. Codoping with ${\mathrm{Li}}^{+}$ ions does not change the emission intensity of water-soluble ${\mathrm{NdF}}_3$ nanoparticles, whereas codoping with ${\mathrm{Ba}}^{2+}$ ions improves the NIR emissions.