Fluorescent and magnetic bifunctional NaLuF4:Yb3+/Tm3+/Gd3+ nanocrystals were synthesized by the solvothermal method and subsequent surface modification. By changing the doping concentration of Gd3+, the shape, size, luminescent properties, and magnetic properties of the nanoparticles can be modulated. These NaLuF4:Yb3+/Tm3+/Gd3+ nanocrystals present efficient blue upconversion fluorescence and excellent paramagnetic property at room temperature. Based on the luminescence resonance energy transfer (LRET), upconversion nanoparticles (UCNPs) were confirmed to be an efficient fluorescent nanoprobe for detecting acriflavine. It is easy to derive the concentration of acriflavine from the Integral Intensity Ratio of Green (emission from acriflavine) to Blue (emission from UCNPs) fluorescent signals. Based on this upconversion fluorescent nanoprobe, the detection limit of acriflavine can reach up to 0.32 μg/mL.

1. Introduction

Rare earth upconversion nanoparticles (UCNPs), which are able to emit high-energy photons under excitation by near-infrared (NIR) light, have three important features: the deep penetration of excitation light to the biotissues, the absence of background fluorescence in biological detection, and no obvious injury to biological tissues [15]. Thanks to various unique intrinsic properties, UCNPs have not only attracted growing attention in the biological chemistry, microarray applications, imaging, but also offered advantages for clinical applications in diagnosis and treatment [612]. Some recent study tuned UC emission through doping with Gd3+ ions [1315]. Meanwhile, due to the large magnetic moment, the upconversion materials doped with Gd3+ can present excellent magnetic behavior. So bifunctional nanocrystals can be achieved by doping a suitable amount of Gd3+ ions.

Acriflavine is a typical antiseptic. It has the form of an orange or brown powder. It may be harmful in the eyes or if inhaled. It is a dye and it stains the skin and may irritate. Acriflavine is also used as treatment for external fungal infections of aquarium fish. In recent years, acriflavine has been confirmed to possess anticancer activity [1618]. However, its potential clinical application is greatly undermined by its uncertain mechanism of activation. It is very important and valuable for detecting traces of acriflavine in vivo.

Fluorescence resonance energy transfer (FRET) is a nonradiative process in which the electronic excitation energy of a donor chromophore is transferred to a nearby acceptor molecule via long-range dipole-dipole interactions. Luminescence resonance energy transfer (LRET) is a specific FRET system with rare earth doped materials as the donor [1921]. Recently LRET has been used to detect molecule binding events and protein [2224]. Here, this LRET process based on upconversion fluorescent donors is developed for detecting organic dye acriflavine.

In this work, based on a simple Gd3+ doping method, shape and size of well controlled NaLuF4 nanocrystals were synthesized. The as-synthesized bifunctional NaLuF4:Yb3+/Tm3+/Gd3+ nanocrystals with highly efficient UC luminescence and excellent paramagnetic behavior can be applied in many fields such as bioseparation, fluorescent, and MRI bioimaging. Here, an upconversion LRET system was applied to detect acriflavine.

2. Experimental

2.1. Materials

All rare earth oxides were of 99.99% purity. Rare earth chloride RE(Cl)3 (RE = Lu, Yb, Tm, and Gd) solutions were prepared by dissolving the corresponding rare earth oxides in hydrochloric acid at a high temperature. All other chemicals were analytical grade and used without further purification.

2.2. Synthesis of NaLuF4:Yb3+/Tm3+/Gd3+ Nanoparticles

Synthesis of NaLuF4:Yb3+/Tm3+/Gd3+ nanoparticles was conducted according to a previously reported procedure as shown in [5, 11].

2.3. Characterization

The shape, size, and uniformity of synthesized UCNPs were measured with a transmission electron microscope (H-7650c) and a high-resolution transmission electron microscopy (JEM 3010). Upconversion luminescence spectra were recorded with a fluorescence spectrometer (Hitachi F-2700), which has a 980 nm laser as the excitation source. A multiple CCD camera (Sony) was used to take the pictures of the upconversion luminescence. Magnetic measurement of the NaLuF4 microcrystals was made using a Lake-shore 7410 vibrating sample magnetometer. Animal tissue imaging with upconversion nanoparticles was conducted using Olympus BX43 fluorescence microscopy. All tests were performed at room temperature.

3. Results and Discussion

3.1. Upconversion Luminescence Properties of NaLuF4:Yb3+/Tm3+/Gd3+ Nanoparticles

To reveal the phase and size control, NaLuF4:Yb3+/Tm3+/Gd3+ nanocrystals synthesized by the solvothermal method were characterized by TEM and high-resolution TEM (HRTEM) as shown in Figure 1. The average diameters of the prepared NaLuF4 nanoparticles doped with 20%Yb3+/0.5%Tm3+, 20%Yb3+/0.5%Tm3+/20%Gd3+, and 20%Yb3+/0.5%Tm3+/60%Gd3+ are determined to be about 320, 180, and 20 nm, respectively. With the increase of Gd3+ content, particle size is gradually reduced. It can be observed from Figures 1(a) and 1(b) that NaLuF4 nanoparticles doped with 20%Yb3+/0.5%Tm3+ and 20%Yb3+/0.5%Tm3+/20%Gd3+ have typical hexagonal crystal facets and good crystallinity, while NaLuF4:20%Yb3+/0.5%Tm3+/60%Gd3+ nanoparticles have a nanoplate morphology and uniform particle size as shown in Figure 1(c). All the three kinds of nanoparticles display regular morphology and good crystalline quality. From the high-resolution transmission electron microscopy (Figure 1(d)), it can be found that the distance between the lattice fringes is 0.31 nm along (0001) orientation in the NaLuF4:20%Yb3+/0.5%Tm3+/60%Gd3+ nanocrystals, which also revealed their highly crystalline nature and structural uniformity.

Under the excitation of 980 nm infrared light, NaLuF4:Yb3+/Tm3+/Gd3+ nanocrystals can emit intense blue light. As shown in Figure 2(a), five emission peaks in the ultraviolet-visible region were assigned to (344 nm), (361 nm), (450 nm), (476 nm), and (646 nm) transitions of the Tm3+ ions, respectively. With the increase of Gd3+ content, the UC emission intensity of NaLuF4 samples increases first and then decreases evidently. When the concentration of Gd3+ ion reaches 20%, the luminescence intensity is the strongest. It can be seen from Figure 2(b) that the intensity ratios of the blue-to-UV emission and blue-to-red emission vary with the Gd3+ concentration. When the content of Gd3+ is equal to 10%, the blue-to-red ratio reaches the maximum value of 81.9 and the blue-to-UV ratio reaches the minimum value of 4.2.

Figure 3 shows the UC luminescence mechanism and population processes in an Yb3+-Tm3+-Gd3+ codoped system. With a 980 nm LD as the excitation source, Yb3+ successively transfers energy to Tm3+ to populate their levels. First, the Tm3+ ions are excited from the ground state 3H6 to the excited state 3H5 via the energy transfer (ET) from neighboring Yb3+ ions to the Tm3+ ions. Subsequent nonradiative relaxation of populates the 3F4 level of Tm3+ ions. In the second-step excitation, Tm3+ ions in the 3F4 state are excited to the 3F2,3 states via the ET from excited Yb3+ ions. The populated 3F2 level may nonradiatively relax to the 3F3 and 3H4 levels. The Tm3+ of 3H4 state can absorb a third pump photon to an upper level 1G4 level, which can yield red (646 nm) and blue (476 nm) emissions upon nonradiative relaxation back to 3F4 and 3H6 levels, respectively. There is another ET process from Yb3+ to Tm3+ ions which may take place to populate the Tm3+ from 1G4 to 1D2, which subsequently resulted in the ultraviolet (UV) and blue emission bands centered at 361 nm and 450 nm, respectively. On the other hand, the Tm3+ ions in the 1D2 state can be excited to the 1I6 state via another ET from excited Yb3+ ions. The UV emissions of ~344 nm can be observed simultaneously via the transitions of , respectively. In addition, the strong ultraviolet emissions demonstrated the high upconversion efficiency in NaLuF4:Yb3+/Tm3+/Gd3+ system since the successive absorption of three or more than three photons involved in the upconversion process of ultraviolet emissions. Some of Tm3+ ions in the 3P2 state transfer energy to levels of Gd3+ through the ET (Tm3+): (Gd3+) [25]. At room temperature, the nonradiative relaxation leads to the population of 6P5/2 and 6P7/2 levels efficiently [26]. Then, the levels of Gd3+ can be populated further. Due to their appropriate energy matching, the ET (Yb3+): (Gd3+) should be the dominant process in populating levels because of the strong absorption of Yb3+ under 980 nm excitation and the high concentration of Yb3+ in the samples. UV and violet emissions also may be observed when transitions happen from the excited states , , levels of Gd3+.

The pumping power dependence of the fluorescent intensity has been investigated. For an unsaturated UC process, the emission intensity is proportional to the power density of the excitation light, and the slope value () is the number of the excitation photons absorbed per emitted photon. The excitation power dependence of the five emission bands of NaLuF4:20%Yb3+/0.5%Tm3+/20%Gd3+ nanocrystals is measured, and treated by Auzel’s method (Figure 4(b)) [27]. Figure 4 shows the power dependence of the UC emission intensities: , 2.85, 2.89, 1.87, and 1.79 for 344, 361, 450, 476, and 646 nm emissions, respectively. Due to the saturation effect [28, 29], the -values of all the five bands are much lower than the theoretical values. When the intermediate excited level has nearly saturated population, it will act as electron reservoir like the ground state and directly transit electrons from its level to the upper one. As a result, it is observed that all the 344, 361, and 450 nm emissions came from three-photon UC processes and two-photon processes are related to the production of 476 and 646 nm emissions.

3.2. Magnetic Behavior

Besides the excellent luminescence characteristic, the Gd3+ doped NaLuF4 nanocrystals also present particular magnetic properties due to the large magnetic moment of Gd3+. Figure 5 shows the relationships between the magnetization of NaLuF4 nanocrystals doped with different Gd3+ ions and the applied magnetic field (from −20 kOe to 20 kOe). It can be seen that both of the two samples present typical paramagnetic behavior, which is mainly because that the seven unpaired inner 4f electrons tightly bound to the nucleus and are effectively shielded by the outer closed shell electrons (5s25p6) from the crystal field. The paramagnetic properties of NaLuF4 nanocrystals can be significantly modulated by doping Gd3+ ions. With the increasing of Gd3+ content, the magnetization is greatly improved. When the concentration of Gd3+ ions increased to 60%, under the applied magnetic field of 20000 Oe, the magnetization of the nanocrystals can reach to 3.9388 emu/g. If we continue to improve the doping concentration of Gd3+ ions, the magnetization of the nanocrystals will be further improved, but the overall intensity of the luminescence will decrease.

3.3. Detection of Acriflavine

Acriflavine is an organic material which is widely used in biochemical research and cell biology experiments. It can emit green light of ~540 nm under the excitation of 470 nm blue light and is preferably dissolved in water, methanol, and ethanol. Acriflavine is known as an antibacterial drug initially approved for the clinical treatment. Recently, increasing evidence has shown that acriflavine exhibits potential antitumor activity and has been recognized as an attractive molecule suitable for cancer chemotherapeutics. On the basis of effective monitoring of acriflavine, we can better explore its mechanism of action and broaden its scope of application in clinic.

The excitation spectra of acriflavine overlaps with the emission spectra of NaLuF4:Yb3+,Tm3+,Gd3+ nanoparticles in blue region. Based on LRET, we can successfully build a sensor system, in which UCNPs play a role of energy donor while acriflavine plays a role of energy acceptor. The basic molecule acriflavine can be quickly captured by the acidic ligand (oleic acid) which lies in the synthesized UCNPs, so that a close nanosystem of UCNPs@Acriflavine is formed. By comparing the relative emission intensities of green emission (acriflavine) and blue emission (UCNPs with emitter Tm3+), the concentration of acriflavine can be correspondingly determined. Here, we show that upconversion NaLuF4:20%Yb3+/0.5%Tm3+/20%Gd3+ nanoprobes are very efficient and viable for detection of acriflavine.

The UC luminescence spectra of UCNPs@Acriflavine system with various concentrations of acriflavine are shown in Figure 6. With the increase of acriflavine concentration, blue emission intensity from the UCNP is reduced, while the emission of acriflavine centered at 490~540 nm becomes stronger gradually. It shows that the energy transfer between UCNP and acriflavine is enhanced with increasing the content of acriflavine. Interestingly, the variation of acriflavine concentration has no obvious influence on the emission of the red from Tm3+ ion. The exciton recombination radiation in acriflavine is highly dependent on LRET from UCNPs to acriflavine. When the concentration reaches 3.2 mg/mL, the blue and UV emission peak of UCNPs disappear while acriflavine shows intense emission centered at 540 nm. Similarly, it can be found that the integrated intensity ratio of green (acriflavine) to blue (UCNPs) emission (IIRGB) is strongly correlated with the concentration of acriflavine. The IIRGB is decreased with the decrease of acriflavine concentration. It is clear that this procedure based on LRET process is very efficient for detecting acriflavine in vitro. Figure 7 shows the pumping power dependence of the fluorescent intensity of UCNPs@Acriflavine system with the concentration of acriflavine fixed at 3.2 mg/mL. The slope values are equal to 2.31 and 2.35 for the observed acriflavine emission centered at 540 and UCNPs emission centered at 646 nm, respectively. This indicates that two-photon upconversion process is involved in both of acriflavine and UCNPs emissions. Using a 980 nm diode infrared power source of 0.34 W/mm2, the acriflavine detection limit can reach 0.32 μg/mL, if properly controlling the concentration of upconversion nanoprobes.

Upconversion fluorescent spectra of UCNPs@Acriflavine with various concentrations of NaLuF4:20%Yb3+/0.5%Tm3+/20%Gd3+ nanoparticles are shown in Figure 8. When the concentration of acriflavine is fixed at 0.32 mg/mL, four emission peaks, centered at 341 nm, 365 nm, 510 nm, and 646 nm, are also observed for this UCNPs@Acriflavine system which can be assigned to the electronic transitions , , and of Tm3+ ions and the emission of acriflavine, respectively. It is clear that both the green emission from acriflavine and blue emission from UCNPs increase with increasing the concentration of upconversion nanocrystals, which indicates that the LRET efficiency has no obvious change with increasing the concentration of upconversion nanocrystals. It can be inferred that the concentration of 25 mg/mL for UCNPs has not reached to the saturation value.

In addition, we used LRET tuned UCNPs for imaging tail fin tissues of crucian carp. The fluorescence imaging of tail fin tissues with UCNPs@Acriflavine under different excitation power is depicted in Figure 9, where the concentration of acriflavine is fixed at 0.32 mg/mL. It is clearly observed that the tail fin tissues exhibited bright cyan light, which is caused by the overlap of blue and green light. Increasing the excitation power, the luminous intensity is gradually enhanced, which indicates that the LRET from UCNPs to acriflavine is remarkably raised with the increase of excitation power.

4. Conclusion

In conclusion, uniform NaLuF4:Yb3+/Tm3+/Gd3+ nanocrystals were synthesized by the solvothermal method and subsequent surface modification. Doping with Gd3+ can easily modulate the particle size of these NaLuF4 nanoparticles. Both the upconversion luminescence and magnetic property of NaLuF4:Yb3+/Tm3+/Gd3+ can be adjusted by controlling the doping level of Gd3+. The Gd3+ doped NaLuF4 nanocrystals have good paramagnetic behavior at room temperature, which provides a simple strategy to merge the two functions into a single phase material. The as-synthesized upconversion nanoprobes have an acidic ligand which can quickly capture the basic dyes acriflavine to form a close UCNPs@Acriflavine system. After absorbing blue emission from NaLuF4:20%Yb3+/0.5%Tm3+/20%Gd3+ by LRET, acriflavine can emit blue light. Based on the intensity ratio of blue to green emission, the detection limit of acriflavine for this upconversion fluorescent nanoprobe can reach up to 0.32 μg/mL. This novel route for detecting the acriflavine molecules will be beneficial to explore its biological activity and broaden its clinic application.

Competing Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.


This research was financially supported by the National Natural Science Foundation of China (Grant nos. 61376076, 61674056, 61675067, 61575062, and 61377024) and the Scientific Research Fund of Hunan Provincial Education Department (Grant nos. 16A072 and 16C0627).