Introduction

In the past decades, functional nano-materials have received much attention due to their wide applications in fluorescence imaging (Medintz et al. 2005; Charbonnière et al. 2006), optoelectronics (Dubertret et al. 2002), and bioengineering applications (Gur et al. 2005; Grätzel 2005). Among them, the II–VI quantum dots have been extensively studied, and many synthetic methods including the high-temperature solution chemistry using the organometallic precursors and solvated metal atom dispersion method are established (Cingarapu et al. 2009). Compared with organic fluorophores, quantum dots have many advantages such as good resistance to photo-bleaching, broad excitation spectra, and narrow defined tunable emission peak (Wu et al. 2003). However, the frequently surface-related non-radiative recombination may reduce the luminescence efficiency significantly and limit the applications of quantum dots in optoelectronics and bioengineering (Hayakawa et al. 2000). Therefore, much attention has been focused on the synthesis of transition/rare earth ion-doped semiconductor nanoparticles and the modulation of their photophysical properties as well as potential applications in photonic and biophotonic fields (Erwin et al. 2005; Raola and Strouse 2002; Bharagava et al. 1994).

Fluorescence resonance energy transfer (FRET) is an excited-state energy transfer process with the initially excited donor chromophore to an acceptor moiety via long-range dipole–dipole interaction when they are in close proximal distance within 1–10 nm (Kong et al. 2014). For the ideal case, the donor should not only have tunable emission for the better control of the spectral overlap with the absorption spectrum of the acceptor but also possess longer excited-state lifetime relative to that of the acceptor (Clapp et al. 2005; Clapp et al. 2004). The efficiency of FRET essentially depends on the extent of spectral overlap, the distance between the donor and acceptor, and their relative orientation of the transition dipole moment as well as the excited-state lifetime of the donor and acceptor (Li et al. 2009). In recent years, there have been many fluorescence probes based on FRET where most of them are confined to systems containing two organic fluorophores of high fluorescence quantum yields (Benniston and Harriman 2006; Coskun and Akkaya 2006). In addition, quantum dots have also been employed as energy donors in FRET experiments. For example, Zhu and coworkers demonstrated energy transfer behavior between CdSe nanocrystals and lucigenin (Dong et al. 2016). However, corresponding studies on the energy transfer behavior of europium ion-doped CdSe quantum dots are very rare (Chowdhury and Patra 2006).

Lanthanide ions possess many rich and particular photophysical properties such as sharp and intense emission bands with corresponding excited-state lifetimes that can reach the millisecond time scale (Bünzli and Piguet 2005). The origin of such long-lived excited state is found to be the nature of f–f electron transitions (Wang et al. 2011). However, their absorption coefficients are very low because f–f transitions are forbidden. So ordinarily, lanthanide ions are conjugated with organic dyes or quantum dots which have large absorption cross section in the region of ultraviolet light and can effectively transfer the energy to the lanthanide ions (Wang et al. 2012). Thus, the synthetic methods of rare earth (RE) ion-doped semiconductor quantum dots with controlled size, structure, and optical properties are extremely important, which have been reported in the previous literatures such as Eu3+-doped CdS and ZnS and Er3+-doped CdS (Chowdhury and Patra 2006; Schmidt et al. 1998). In our work, RE ions are incorporated into the CdSe nanocrystals or adsorbed at a nanocrystal surface which could modulate the optical properties of hybrid quantum dots. CdSe quantum dots are used as the antenna materials to sensitize and protect RE ions as well as address their drawback of weak molar absorption coefficients (often less than 1 M−1 cm−1) (Ghadiali et al. 2010). But there is still a challenge to prepare the lanthanide cation-doped CdSe quantum dots with tunable luminescence in the wide visible region (Sarkar et al. 2013). On the other hand, β-diketone is well known to be a good chelating group to sensitize the luminescence of rare earth ion in which β-diketone is used to reinforce the absorption ability and transfer the absorption energy to RE ions with high efficiency (Qiao and Yan 2009). As a result, a large number of RE ions and their complexes give visible (Sm3+, Eu3+, Tb3+) or infrared (Nd3+, Ho3+, Yb3+, Er3+) luminescence, which mainly depend on the nature of RE ions (Lenaerts et al. 2005; Zhang et al. 2010, 2014). For example, Luwang and coworkers employed 2-thenoyltrifluoroacetone (TTA) as an example of β-diketone to successfully sensitize Eu3+ ion-doped SrF2 nanoparticles (Ghosha and Luwang 2015). Therefore, it is of great interest to study the luminescence of such lanthanide-doped CdSe quantum dots, which is decorated with sensitizing organic molecule 2-thenoyltrifluoroacetone (TTA).

Recently, several synthetic approaches have been employed to prepare spherical nanocrystals. For example, Hines and Scholes used the hot-injection approach to prepare non-water-soluble colloidal PbS quantum dots (QDs) (Hines and Scholes 2003). Yu and coworkers employed a non-injection and low -temperature approach to prepare colloid photoluminescent nanocrystals with narrow bandwidth (Liu et al. 2009). It has also mentioned that for the hot-injection approach, the particle size is modulated mainly by the injection temperature while for the non-injection approach, the particle size is controlled mainly via raising temperature, namely, the growth of particles can be easily modulated by raising temperature. To the best of our knowledge, the preparation of luminescence CdSe:Eu3+ hybrid nanocrystals by using non-injection method is very rare. In this paper, CdSe:Eu3 hybrid nanocrystals are successfully prepared by the non-injection one-pot synthesized method and the size, nanocrystal structure, composition, and the optical property are investigated. Interestingly, the hybrid materials possess uniform size through controlling the synthetic conditions. Upon the variation of the feed ratios and reaction temperatures, strong and tunable fluorescence from 490 to 650 nm can be observed. Furthermore, the luminescence of europium ion drastically increases upon decoration with 2-thenoyltrifluoroacetone (TTA). The mechanism for fluorescence enhancement is proposed to be energy transfer from CdSe host to the europium ion. Herein, the synthesis and tunable luminescence properties of CdSe:Eu3+ hybrid nanocrystals are described.

Materials and methods

Materials

1-Octadecene (90%) and oleic acid (90%) were purchased from Aldrich. Selenium powder (99.5%) was purchased from Alfa Aesar. Cadmium acetate dehydrate [Cd(OAc)2·(H2O)2], toluene, methanol, and acetone were all analytical grade. All reagents were used as received without further purification. Europium trichloride hexahydrate [EuCl3·6H2O] was prepared from europium oxide [Eu2O3] (0.8 g) treated with hydrochloric acid (36.5 wt%, 1.6 mL). Subsequently, the solvent was removed by heating and then addition of deionized water (1 mL); the solvent was removed again. Similarly, deionized water was added to the above residue once again; the solvent was removed again. The process was repeated for three times to remove the residue hydrochloric acid. At last, the residue was redissolved in deionized water and adjusted pH to 6 for further application.

Synthesis of CdSe quantum dots (CdSe QDs)

The CdSe QDs were synthesized according to previous literatures reported by Peng and coworkers with a slight modification (Qu and Peng 2002). The solution containing Cd(OAc)2·(H2O)2 (100 μmol), oleic acid (0.8 mL), and octadecene (4.2 mL) was degassed, heated up to 120 °C, and then placed for 1 h at this temperature with continuous stirring. After that, the mixture was further heated to 280 °C and placed at this temperature for 10 min. Subsequently, a stock solution of selenium powder (3.9 mg, 50 μmol) dispersed in octadecene (2 mL) was quickly injected, and the growth reaction was performed at this temperature for 10 min. The temperature of the solution was then dropped to 100 °C below by using the method of water bath. The resultant nanocrystals were precipitated upon addition of excess acetone, rinsed with methanol, and then dispersed in toluene (10 mL).

Synthesis of CdSe:Eu3+ nanocrystals

The mixture of EuCl3·6H2O (20 μmol), Cd(OAc)2·(H2O)2 (80 μmol), selenium powder (3.9 mg, 50 μmol), oleic acid (0.8 mL), and octadecene (4.2 mL) was stirred in a flask (50 mL), degassed and heated up to 120 °C under vacuum condition, and then placed at this temperature for 1 h to eliminate water. After that, the resulting cloudy homogeneous mixture was further heated to 160 °C and placed at this temperature for 10 min. The temperature of the solution was then dropped to 100 °C below by using the method of water bath. The CdSe:Eu3+ hybrid nanocrystals were precipitated upon addition of excess acetone, rinsed with methanol, and then dispersed in toluene (10 mL). Using the same method and condition, only by changing the heated temperature from 160 to 120, 200, 240, and 280 °C, other nanocrystals were prepared.

Preparation of 2-thenoyltrifluoroacetone-capped CdSe:Eu3+ nanocrystals or EuCl3·6H2O

The mixture of CdSe:Eu3+ nanocrystals (20 μmol) and excess 2-thenoyltrifluoroacetone (TTA) was stirred in toluene under nitrogen. Subsequently, the solution was heated up to 55 °C and placed at this temperature for 10 h. When cooled to room temperature, the solution was placed for further application. Using the same method, EuCl3·6H2O was also modified by TTA.

Cellular toxicity test

Human epithelial type 2 (Hep-2) cells (104 cells/100 μL) were cultured for 24 h in an incubator with 5% CO2 at 37 °C. After that, the culture medium was replaced with Dulbecco’s modified Eagle’s medium (DMEM) containing CdSe:Eu3+ nanocrystals at different doses (0, 10, 20, 30, 40, 50, 60 μg mL−1) and cultured for another 24 h. Subsequently, 20 μL of MTT solution (5 mg mL−1) was added to every cell well. The cells were incubated for 4 h and removed the culture medium containing MTT, then 150 μL DMSO was added. The resulting mixture was shaken for about 5 min at room temperature. The optical density (OD) of the mixture was measured at 490 nm. The cell viability was estimated according to the following equation: cell viability (%) = ODtreated/ODcontrol × 100%, where ODtreated and ODcontrol were obtained in the presence or in the absence of CdSe:Eu3+ nanocrystals, respectively.

Cellular imaging

The Hep-2 cells were cultured in DMEM containing 10% fetal bovine serum and 1% penicillin/streptomycin using a 96-well plate. Suspensions (50 μg mL−1) of CdSe:Eu3+ nanocrystals from the stock solution were prepared with Dulbecco’s phosphate buffer saline and DMSO mixed solution (DPBS/DMSO = 1:1 v/v). After sonication for 5 min to ensure complete dispersion, 10 μL of suspension was added to the well of the plate and incubated at 37 °C in a 5% CO2 incubator for 32 h. Prior to fixation of Hep-2 cells on the slide for inspection with confocal fluorescence microscopy, the excess CdSe:Eu3+ nanocrystals were removed by washing three times with a DPBS and DMSO mixed solution (1:1 v/v).

Characterization

UV–vis absorption spectra were obtained by using a Lambda 800 UV–Vis spectrophotometer. Photoluminescence (PL) experiments were performed with a Shimadzu RF–5301 PC spectrophotometer. Structural and compositional characterizations were performed by transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDX). TEM was recorded by JEOL-2010 electron microscopy operating at 200 kV. High-resolution TEM and EDX were characterized by TECNAI F20 TEM operating at 200 kV. X-ray diffraction (XRD) data were collected on a Siemens D–5005 X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å). The confocal microscopy images were taken using an Olympus Fluoview FV1000. All measurements were performed at room temperature.

Results and discussion

CdSe:Eu3+ hybrid nanocrystals (CdSe:Eu3+ NCs) were prepared by the non-injection one-pot method using oleic acid as capping agents. This method is easier relative to multiple injections. The use of oleic acid has two main advantages: (1) oleic acid has high boiling point (350~360 °C) and is stable when the reaction temperature is high, which decreases the formation of by-products and (2) oleic acid has an interaction with metal ions, which facilitates the preparation of CdSe:Eu3+ hybrid nanocrystals. To modulate the optical properties of CdSe:Eu3+ hybrid nanocrystals, the reaction is performed at different temperatures (120, 160, 200, 240, 280 °C). When samples are prepared at 120 °C, very weak absorption peak at about 480 nm is observed. As the synthetic temperature is increased from 120 to 160 °C, their characteristic absorption band with the peak value at about 540 nm is observed. Further increasing reaction temperature from 160 to 280 °C results in redshift of the absorption peak from 540 to 630 nm (Fig. 1a). To further understand their optical properties, the emission spectra are investigated and shown in Fig. 1b. Similar to the UV-vis absorption spectra, very weak fluorescence peak positioned at 490 nm is observed when the reaction is performed at 120 °C. Upon increasing reaction temperatures from 160 to 280 °C, the emission spectra of CdSe:Eu3+ NCs show redshift with maximum emission wavelength shifting from about 550 to 630 nm. The results suggest that emission spectra can be modulated by the variation of reaction temperatures. In addition, the relative quantum yields of CdSe:Eu3+ NCs are measured by using Rhodamine 6G as the reference (water as solvent; QY = 0.78). The quantum yields of resultant samples are 18% for CdSe quantum dots and 11% (160 °C), 15% (200 °C), 9% (240 °C), and 6% (280 °C) for CdSe:Eu3+ NCs. These results indicate that CdSe:Eu3+ NCs give different emission intensity and tunable luminescence from green to red upon the variation of reaction temperatures from 120 to 280 °C.

Fig. 1
figure 1

a UV-vis absorption spectra of CdSe:Eu3+ NCs prepared at different temperatures: (1) 120 °C, (2) 160 °C, (3) 200 °C, (4) 240 °C, (5) 280 °C. b Emission spectra of CdSe:Eu3+ NCs prepared at different temperatures: (1) 120 °C, (2) 160 °C, (3) 200 °C, (4) 240 °C, (5) 280 °C

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To further understand the effect of Eu3+ ion on the UV-vis absorption and emission spectra of CdSe, the relationship between the maximum absorption wavelength and emission wavelength of CdSe and CdSe:Eu3+ NCs is investigated. UV-vis absorption and emission spectra of CdSe and Eu3+ ion are shown in Figure S1 and Figure S2. Based on these results, the relationship between the maximum absorption wavelength and emission wavelength of CdSe and CdSe:Eu3+ NCs at 200 °C and other temperatures (120, 160, 240, 280 °C) is summarized in Fig. 2. Similar to those of CdSe quantum dots, UV-vis absorption and emission spectra of CdSe:Eu3+ NCs show redshift phenomenon with increasing reaction temperatures. Moreover, their UV-vis absorption wavelength and emission wavelength exhibit about 40-nm redshift compared with those of CdSe quantum dots when the reaction temperature changes from 160 to 280 °C. Furthermore, the difference between the maximum absorption and emission wavelengths of CdSe:Eu3+ NCs and CdSe quantum dots is investigated. The hybrid CdSe:Eu3+ NCs give smaller difference relative to CdSe quantum dots when the reaction temperature changes from 160 to 280 °C. These results suggest that Eu3+ ion is successfully incorporated into the CdSe nanocrystal lattice and has affected the fluorescence of CdSe effectively when reaction temperatures change from 160 to 280 °C.

Fig. 2
figure 2

The relationship between maximum absorption and emission wavelengths of CdSe quantum dots and CdSe:Eu3+ NCs at different reaction temperatures (120, 160, 200, 240, 280 °C)

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For the hybrid nanocrystals, it is important to characterize the size, crystal structure, and composition. The size and morphology of CdSe:Eu3+ NCs are characterized using TEM. Their TEM images and size distribution are shown in Figure S3 and Figure S4. As shown in Figure S3 and Figure S4, the nanocrystals show narrow size distribution when reaction temperature changes from 160 to 280 °C and have no obvious aggregation formed. Moreover, CdSe:Eu3+ NCs are spherical, and the particle size increases from 2.5 to 4.4 nm with increasing reaction temperatures from 160 to 280 °C, which has also been observed in the previous literature (Liu et al. 2009). After that, their size distribution is further characterized using a high-resolution TEM (HRTEM) and shown in Fig. 3. In the HRTEM images, nanocrystals show well crystalline and their size is consistent with the result of TEM images. In addition, the lattice parameter of 3b–3d is measured to 0.26 nm, and the distance is consistent with the (111) plane of the tetragonal CdSe structure, which is in agreement with the X-ray diffraction pattern. To further understand the crystal structure of hybrid materials, X-ray diffraction is performed and shown in Fig. 4. CdSe:Eu3+ NCs show cubic crystal structure with diffraction peaks of 220 and 311, which match well with characteristic cubic structure of host CdSe nanocrystals. With the increase of the temperature, 220 and 311 diffraction peaks show slight difference. Moreover, an additional peak at around 30o (30.32o) appears for the nanocrystals prepared at 200 and 240 °C, which indicates that probable small EuSe2 particles occur, which has been reported in the previous literatures (Adachi et al. 2008; Hasegawa et al. 2008; Raola and Strouse 2002). These results suggest that X-ray diffraction patterns show a slight change. In addition, these results also confirm that Eu3+ ions are distributed within the CdSe crystal lattice. The ionic radius of Eu (0.95 Å) is close to that of Cd2+ ion (0.98 Å), and the high level of Cd vacancies is used to balance the lattice charge. The two factors facilitate Eu ion to substitute the Cd ion in the CdSe crystal lattice. On the other hand, the compositions of CdSe:Eu3+ NCs are characterized by energy dispersive X-ray spectroscopy (EDX) and shown in Fig. 5. Compared with the spectrum of CdSe, CdSe:Eu3+ NCs show several peaks of Eu, which suggests that Eu and CdSe nanocrystals have been combined successfully. Moreover, CdSe:Eu3+ NCs EDX spectra at different reaction temperatures (160, 200, 240, 280 °C) are measured and shown in Figure S5. The Eu and Cd atomic ratios are 0.14 ± 0.02, 0.18 ± 0.03, 0.2 ± 0.03, and 0.22 ± 0.04 when reaction temperature changes from 160, 200, and 240 to 280 °C, which are calculated by the equipment software within the selected area with the scanning pixel of 20 multiplying by 20. It could be seen that the atomic ratios of Eu and Cd in CdSe:Eu3+ NCs show a gradual increase upon increasing reaction temperatures from 160 to 280 °C.

Fig. 3
figure 3

The high-resolution TEM images of CdSe:Eu3+ NCs prepared at different reaction temperatures. a 160 °C. b 200 °C. c 240 °C. d 280 °C

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Fig. 4
figure 4

The XRD patterns of the as-prepared CdSe:Eu3+ NCs with the variation of reaction temperatures: (1) 160 °C, (2) 200 °C, (3) 240 °C (4) 280 °C

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Fig. 5
figure 5

EDX spectra of a CdSe quantum dots and b CdSe:Eu3+ NCs

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Synthetic conditions including the amount of Se powder, molar ratio of oleic acid (OA) and metal ions (Eu, Cd), and the amount of EuCl3·6H2O were studied. The amount of Se powder is an important consideration in the synthetic process of CdSe:Eu3+ NCs. When the amount of Se powder is changed from 0.025 to 0.1 mM, the emission peak shows redshift behavior with a peak position changing from 620 to 660 nm (Fig. 6a). The redshift behavior of CdSe:Eu3+ NCs is ascribed to the quantum confinement effect, which could be proved by transmission electron microscopy. The particle size distribution is calculated and shown in Figure S6. When the amount of Se powder changes from 0.025 to 0.1 mM, the average size increases from 2.2 ± 0.3 to 3.9 ± 0.4 nm, which suggests more Se atoms exist in the host lattice and increase the particle size. The molar ratio of ligand and metal ions is another important factor for spectral modulation. When the molar ratio of OA and metal ions increases from 1 to 8, maximum emission wavelength of CdSe:Eu3+ NCs exhibits redshift behavior with a peak position changing from 550 to 660 nm (Fig. 6b). Moreover, their size distribution is studied and shown in Figure S6. The average particle size increases from 1.8 ± 0.3 to 3.8 ± 0.5 nm when the molar ratio of OA and metal ions changes from 1 to 8. In addition, when the total molar number of Eu3+ and Cd2+ is fixed at 0.1 mmol, their emission behavior is investigated by modulating the molar ratio of Eu3+ and Cd2+ ranging from 0 mmol:0.1 mmol to 0.1 mmol:0 mmol. The results show that with the changing of Eu3+ concentration from 0.005 to 0.025 mM, the emission peak exhibits redshift phenomenon ranging from 580 to 650 nm (Figure S7). To our knowledge, the covalent radius of Eu (1.85 Å) is larger than that of Cd (1.48 Å), which may facilitate the increase of CdSe:Eu3+ NCs size. Based on the consideration of the quantum confinement effect on the luminescent properties, the redshift behavior of maximum emission wavelength is proposed to be the result of increasing the particle sizes, which have been proved by the results of TEM images and the size distribution (Fig. 6 and Figure S6–S7).

Fig. 6
figure 6

a The relationship between the maximum emission wavelength and the amount of Se as well as TEM imaging of CdSe:Eu3+ NCs when the concentration of Se is a-1 0.025 mM and a-2 0.1 mM. b The relationship between the maximum emission wavelength and the ratio of OA and the sum of Eu and Cd as well as TEM imaging of CdSe:Eu3+ NCs when the amount of OA is b-1 130 μL and b-2 390 μL

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In our as-prepared CdSe:Eu3+ hybrid nanocrystals, europium ion is embedded in CdSe host materials that facilitate the protection of europium ion luminescence and decrease environmental quenching. Since the f–f transition of Eu3+ ion is forbidden, the molar absorption coefficients and luminescence intensity are poor. In order to enhance emission intensity of such hybrid nanocrystals, one effective way is performing decoration sensitizing organic molecules such as TTA on their surface. Based on these considerations, we use CdSe:Eu3+ NCs prepared at 240 °C as a model to study the optical properties and further understand the effect of TTA on the luminescent properties of CdSe:Eu3+ NCs. As expected, the absorption spectrum of CdSe:Eu3+ NCs shows a slight blueshift upon decoration with TTA (Fig. 7a), which suggests the interaction between TTA and Eu3+ ion. By extrapolating the linear region of the plot of the absorbance square versus energy, the band gap of CdSe:Eu3+ NCs with and without TTA is estimated to be 2.0 and 1.97 eV, respectively. Upon excitation at 430 nm, CdSe:Eu3+ NCs show strong fluorescence and blueshift from 635 to 612 nm upon modification with TTA (Fig. 8a), consistent with the result of the band gap of CdSe:Eu3+ NCs. The fluorescence positioned at about 635 nm is ascribed to the luminescence of CdSe:Eu3+ NCs, while the fluorescence centered at 612 nm is tentatively ascribed to that of Eu3+ ion, mixed with ligand-to-metal charge transfer emission. In addition, the emission intensity of CdSe:Eu3+ NCs is greatly increased relative to that of Eu3+ ion upon decoration with TTA (Fig. 8b). The mechanism for fluorescence enhancement is proposed to be energy transfer from CdSe quantum dots to Eu3+ ion, and energy transfer efficiency is about 89%, which has been reported in the previous literature (Reisfeld et al. 2000). Energy transfer behavior from CdSe quantum dots to Eu3+ ion is further supported by the excitation spectra of CdSe:Eu3+ hybrid nanocrystals and Eu3+ ion. The excitation spectra of CdSe:Eu3+ NCs and Eu3+ ion are shown in Fig. 7b, c. For clarity, the magnified Fig. 7c is shown in Figure S8. As indicated, upon modification of CdSe:Eu3+ NCs with TTA, the excitation spectrum of CdSe:Eu3+ NCs is the sum of CdSe:Eu3+ hybrid nanocrystal lack of TTA and Eu3+ ion, which confirms that energy transfer occurs from CdSe quantum dots to Eu3+ ion upon modification with TTA. The size of CdSe:Eu3+ NCs upon modification of TTA has been characterized by TEM (Fig. 9). The size distribution is investigated and shown in Figure S9. Compared with that of CdSe:Eu3+ NCs without TTA, the average size of CdSe:Eu3+ NCs shows slight changes from 3.6 to 3.9 nm upon decoration with TTA. The result suggests the quantum confinement effect has an almost negligible effect on the spectral properties and further confirms energy transfer occurs from CdSe quantum dots to Eu3+ ion upon decoration with TTA. Energy transfer mechanism from CdSe quantum dots to Eu3+ ion is further supported by the previous literatures about CdS: Eu3+ hybrid nanocrystals (Reisfeld et al. 2000). It is reported that upon excitation of CdS host, the energy from non-radiative recombination of electron-hole pairs could be transferred to the high-lying energy levels of the Eu3+ ion. Another explanation is that the photo-generated electron is trapped in a surface level of CdS nanoparticles, and the trapped electron has strong interaction with a europium ion and then transfers energy to europium ion. In addition, other reports suggest that site symmetry of ions plays the most important role in the modifications of radiative and non-radiative relaxation mechanisms (Chowdhury and Patra 2006). These results indicate that our work proposes a novel approach for the modulation of optical properties of Eu3+ in a nanocrystal system and this strategy could be expected to have potential applications in biological labeling, immunoassays, and optical sensing.

Fig. 7
figure 7

a UV-vis absorption spectral changes of CdSe:Eu3+ NCs upon modification with 2-thenoyltrifluoroacetone (TTA). b Excitation spectral changes of CdSe:Eu3+ NCs upon modification with TTA. c The excitation spectrum of Eu3+ ion upon modification with TTA

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Fig. 8
figure 8

a Emission spectral changes of CdSe:Eu3+ NCs upon modification with TTA. b Emission spectra of CdSe:Eu3+ NCs and Eu3+ ion upon modification with TTA

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Fig. 9
figure 9

The TEM image of CdSe:Eu3+ NCs upon modification with TTA

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To further understand the potential applications of CdSe:Eu3+ NCs in a biological system, cellular toxicity and cellular imaging in cells are investigated. Based on the consideration, we use CdSe:Eu3+ NCs prepared at 240 °C and Hep-2 cells as models to study the potential cellular toxicity and cellular imaging. In this study, an MTT and apoptosis assay are employed to evaluate the toxicity of CdSe:Eu3+ NCs. As shown in Fig. 10, the Hep-2 cells remain more than 80% viability after they are incubated with CdSe:Eu3+ NCs with different concentrations (0, 10, 20, 30, 40, 50, 60 μg mL−1) for 24 h. The result suggests that CdSe:Eu3+ NCs have low acute toxicity. In addition, the cell internalization and intracellular distribution of CdSe:Eu3+ NCs are evaluated with confocal laser fluorescence microscopy. Figure 11 shows the bright field, confocal fluorescence, and overlaid imaging of Hep-2 cells incubated with CdSe:Eu3+ NCs for 32 h. As revealed in the bright field, Hep-2 cells incubated with CdSe:Eu3+ NCs maintain their normal morphology, suggesting good biocompatibility at this dose and incubation time. Fluorescence imaging irradiated with 488 nm shows red fluorescence within Hep-2 cells, which reveals the uptake behavior of Hep-2 cells. These results reveal that our CdSe:Eu3+ NCs could be used as cellular imaging and have potential applications in biological labeling and sensing.

Fig. 10
figure 10

Viability of Hep-2 cells after incubation with different concentration of CdSe:Eu3+ NCs in the cell medium for 24 h as determined by an MTT assay

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Fig. 11
figure 11

a Bright field, b confocal fluorescence, and c overlaid imaging of Hep-2 cells captured by laser scanning confocal microscopy

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Conclusions

Fluorescent CdSe:Eu3+ hybrid nanocrystals were prepared by non-injection and one-pot method using oleic acid as a capping agent. The use of oleic acid not only decreases the formation of byproducts but also facilitates the preparation of CdSe:Eu3+ nanocrystals. The as-prepared nanocrystals have been characterized by transmission electron microscopy, X-ray diffraction, and energy dispersive X-ray spectroscopy (EDX), and their UV-vis absorption spectra as well as photoluminescence spectra are investigated. The results confirm that europium ion is successfully incorporated into CdSe crystal lattice. In addition, the nanocrystals show tunable fluorescence from green to red through the variation of feed ratios or reaction temperatures. Upon modification of CdSe:Eu3+ nanocrystals with TTA, efficient energy transfer occurs from CdSe host to europium ions and energy transfer efficiency is about 89%. Energy transfer behavior has been proved by the emission and excitation spectra of CdSe:Eu3+ nanocrystals and Eu3+ ion and the average size of CdSe:Eu3+ nanocrystals with and without TTA and supported by the energy transfer behavior of the previous CdS: Eu3+ hybrid nanocrystals. In addition, our CdSe:Eu3+ nanocrystals have low cellular toxicity and could be used as fluorescence imaging for Hep-2 cells. Our work proposes a novel approach for the modulation of optical properties of Eu3+ in a nanocrystal system, and this strategy could be expected to have potential applications in biological labeling, immunoassays, and optical sensing.