Controlling red upconversion luminescence in Gd₂O₃:Yb³⁺–Er³⁺ nanoparticles by changing the different atmosphere

Abstract

Lanthanide (Ln³⁺)-doped nanoparticles that produce strong red upconversion (UC) luminescence are desirable for biological applications. For nanoparticles, modification of their electronic properties and stabilization of their crystallographic phase are crucial for tuning their luminescence behaviors. Here, Yb³⁺–Er³⁺ doped Gd₂O₃ nanoparticles have been synthesized by calcining Gd₂O₂S in nitrogen (O₂-free) and ambient air (∼21% O₂), respectively. Various approaches including X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman spectroscopy and thermo-gravimetric and differential thermal analyses (TG-DTA) were used to characterize the structure, morphology and phase formation mechanism of the nanoparticles. Upconversion luminescence properties were investigated by emission intensity vs. excitation power (double logarithmic relationship) and temperature dependent emission spectroscopy. The different splitting transition probabilities of the cubic and/or monoclinic phase, the probability of multiphonon relaxations and cross relaxations between Er³⁺ ions are responsible for the efficient red emission in the cubic Gd₂O₃ nanoparticles. More importantly, the lower phonon energy of the pure cubic sample has good thermal stability in the current detectable range of 20–100 °C. These results imply that the cubic Gd₂O₃ upconversion nanoparticles may have multiplexing functionality in bio-imaging.

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Introduction

Lanthanide-doped upconversion nanoparticles (UCNPs) are a promising optical functional species for the scientific community due to their unique spectroscopic properties and their potential applications in different fields.^1–12 Compared to semiconductor quantum dots and organic fluorophores, properties such as a high detection sensitivity, minimum photo-damage to biological materials, weak auto-fluorescence and a high penetration depth in living organisms or environmental samples allow UCNPs to be regarded as ideal luminescent candidates in the fields of bio-photonics and biomedical imaging.^13–19 In recent years, the development of inorganic functional nanoprobes with integrated magnetic and fluorescent characteristics has been receiving great attention. These materials hold great prospects for multiplexed bio-imaging, such as diagnostic analysis, real-time imaging and fluorescent labeling.²⁰ However, it is complex and difficult to bind two or more kinds of NPs with separate magnetic and optical functionalities into one nanosystem. To this end, it’s a better choice to develop effective multifunctional NPs without adding any other moieties. From this point of view, Gd-based NPs (Gd₂O₂S, NaGdF₄, etc.) are promising multifunctional bio-probes that integrate optical and magnetic properties in a single phase.²¹

During the past decades, the Yb³⁺–Er³⁺ sensitizer–activator pair has been recognized as an ideal lanthanide dopant for efficient upconversion luminescence. This pair usually shows two strong peaks of green emission which greatly limits the imaging penetration depth in tissues.^22,23 Red emission is relatively attractive for the optical imaging and simultaneous tracking of multiple biological species due to the wavelengths that are located in the first biological “optical window”.^24,25 Scientists are devoted to achieving bright red upconversion luminescence using diverse approaches, e.g., choosing host lattices, optimizing dopant concentrations, doping heterogeneous ions (Mn²⁺, Fe³⁺, etc.) and employing photonic crystals.^26–33 However, it remains a challenge to generate bright and pure red upconversion using a simple synthesis process and with multimodal functionality.

Herein, we report a succinct approach to control the red upconversion luminescence of Ln-doped (Ln = Er³⁺, Yb³⁺) Gd₂O₃ NPs which possess magnetic functionality due to the Gd³⁺ ion. We synthesized the Gd₂O₃ NPs through calcining the Gd₂O₂S precursor with limited oxygen access. The structure, the formation process of the NPs, and the upconversion spectroscopic characteristics were investigated in detail. Comparative analysis of the Er³⁺ splitting transitions showed that the efficient cross relaxations between the Er³⁺ ions dominated the strong quasi-pure red upconversion luminescence in the Gd₂O₃–O₂ sample. It is expected that these NPs have potential applications in multimodal biomedical imaging because of their compatible optical and structural properties.

Experimental

Materials and methods

GdCl₃·xH₂O (99.99%), YbCl₃·6H₂O (99.9%), ErCl₃·6H₂O (99.9%), 1-octadecene (ODE, 90%), oleic acid (OA, 90%), oleylamine (OM, 80–90%), sulfur (≥99.5%) and methanol (≥99.9%) were all purchased from Sigma-Aldrich. Acetylacetone was obtained from Aladdin. NaOH (AR), ethanol (AR) and cyclohexane (AR) were supplied by Sinopharm Chemical Reagent Company. All chemicals were used without further purification. Doubly distilled water was used in the experiment.

Synthesis of Gd₂O₂S:9% Yb³⁺,1% Er³⁺ nanoparticles

The synthesis of the Gd₂O₂S:9% Yb³⁺,1% Er³⁺ nanoparticles was similar to the previously reported protocol with some minor modifications.³⁴ Ln(acac)₃·xH₂O (0.5 mmol), Na(acac)₃·xH₂O (1.0 mmol), S (1.0 mmol), OA (2.5 mmol), OM (17 mmol) and ODE (20 mmol) were added in a three-neck flask (50 mL) at room temperature. The mixture was heated to 125 °C under vacuum for 3 h to obtain an optically clear solution. Then the temperature was raised to 315 °C at a rate of 20 °C min⁻¹ under a highly purified Ar atmosphere and maintained at this temperature for 1 h. The solution was cooled down to room temperature, and the Gd₂O₂S:9% Yb³⁺,1% Er³⁺ nanoparticles were precipitated using absolute ethanol, followed by centrifugation and dried at 60 °C for 12 h.

Synthesis of Gd₂O₃:9% Yb³⁺,1% Er³⁺ nanoparticles

The Gd₂O₂S:9% Yb³⁺,1% Er³⁺ nanoparticles were used as the precursors for the preparation of the Gd₂O₃:9% Yb³⁺,1% Er³⁺ nanoparticles. The Gd₂O₂S:9% Yb³⁺,1% Er³⁺ precipitate was put into a small crucible, then calcined at 800 °C for 2 h under an N₂ (or ambient air) gas atmosphere with the heating rate of 4 °C min⁻¹, and then naturally cooled down to room temperature.

General characterization

The X-ray diffraction (XRD) pattern of Gd₂O₃:9% Yb³⁺,1% Er³⁺ was recorded using a Bruker D8 Advance X-ray diffraction system with Cu-Kα (1.5405 Å) radiation within a 25–65° range of 2θ. The shape and morphology of the samples were characterized using a SU8010 FE-SEM scanning electron microscope (HITACHI, Japan) and a FEI Tecnai G2 F20 S-WTINE transmission electron microscope. The TG-DTA curves were recorded using TGA/DSC 1LF1600 (METTLER-TOLEDO, Swit) with a rate of 4 °C min⁻¹ under N₂ and ambient air. The Raman spectroscopy was performed using a 476G72 inVia-Reflex Raman Microscope (Renishaw, UK) under 785 nm excitation. The upconversion luminescence spectra were obtained with a PL3-211-P spectrometer (HORIBA JOBIN YVON, America) using a power-tunable 980 nm diode laser as the excitation source. For temperature dependent emissions, a TAP-02 temperature controller (ORIENT KOJI, China) was equipped to the spectrometer.

Results and discussion

Phase and morphology

Fig. 1(a) and (b) show the XRD patterns of the Gd₂O₃ NPs after calcinations in nitrogen (O₂-free) and ambient air (∼21% O₂), and the corresponding samples are labelled as Gd₂O₃–O₂-free and Gd₂O₃–O₂, respectively. Gd₂O₂S NPs, which were prepared via a wet-chemical route, were used as precursors for the calcinations (see the ESI†). After annealing at 800 °C, the Gd₂O₃–O₂-free sample presented two crystal phases (Fig. 1(a)). The peaks can be directly indexed as cubic Gd₂O₃ (JCPDS no. 43-1014) and monoclinic Gd₂O₃ (JCPDS no. 42-1465). The XRD pattern of Gd₂O₃–O₂ is shown in Fig. 1(b), and the peaks can be indexed as a pure cubic phase of Gd₂O₃ (JCPDS no. 43-1014). TEM was utilized to characterize the morphology and structure of the NPs. From Fig. 1(c) and (d), the morphologies of the Gd₂O₃ NPs are slightly different due to the existence of the monoclinic phase in the Gd₂O₃–O₂-free sample. It can be seen that the obtained Gd₂O₃ displayed a nanosized polyhedron structure. The existence of monoclinic Gd₂O₃ NPs in the Gd₂O₃–O₂-free sample was confirmed by selective particle observation using high-resolution TEM, in which an interlayer spacing of 0.304 ± 0.01 nm was in good agreement with the d spacing of the (401) lattice plane of the monoclinic phase (Fig. 1(e)). The random observation of Gd₂O₃–O₂ revealed the unitary cubic phase, e.g., the interlayer spacings of 0.312 nm, 0.411 nm, 0.185 nm and 0.254 nm are in accordance with the lattice spacing of the (222), (211), (433) and (411) planes of the cubic phase, respectively (Fig. 1(f)). These observations are in accordance with the XRD results.

Fig. 1 Structures and morphologies of the nanoparticles. XRD patterns of (a) Gd₂O₃–O₂-free and (b) Gd₂O₃–O₂. TEM images of (c) Gd₂O₃–O₂-free and (d) Gd₂O₃–O₂. HRTEM images of (e) Gd₂O₃–O₂-free and (f) Gd₂O₃–O₂.

Atmosphere controlled crystal formation

To investigate the formation dynamics of Gd₂O₃ from Gd₂O₂S, TG-DTA was performed. Fig. 2 shows the TG-DTA curves of Gd₂O₂S heated at a rate of 4 °C min⁻¹ in different atmospheres. In the case of the O₂-free atmosphere (Fig. 2(a)), the formation of Gd₂O₃ can be roughly divided into three stages. The first stage with a weak weight loss is related to the evaporation of physically adsorbed and chemisorbed water in the temperature range of 30 °C to 300 °C.¹⁷ The endothermic peaks at about 318 °C and 428 °C are accompanied by a second weight loss of 32% between 300 °C and 480 °C in the TG-DTA curves, which is in accordance with the decomposition of the crystal water and oxidation of the surface organic matter, respectively.³⁵ The weight loss (5%) above 480 °C indicates the decomposition from the intermediate phase to the oxide.³⁶ The strong exothermic peak is attributed to the decomposition of Gd₂O₂S at 580 °C. The incomplete burning of decomposed organic materials produced the endothermic peak at 680 °C.³⁷ At the same time, the residual organic carbon reacted with the decomposed sulfur, which led to the conditions for the phase transition or induced the formation of the monoclinic phase Gd₂O₃. The reactions can be described by the following equations:

C + 2S → CS₂↑ (2)

Fig. 2 TG-DTA curves of Gd₂O₂S synthesized under different atmospheres. (a) Sample calcined in N₂ at a heating rate of 4 °C min⁻¹ and (b) sample calcined in ambient air at a heating rate of 4 °C min⁻¹.

The reaction temperature noticeably decreases under aerobic conditions, which is shown in Fig. 2(b). It may be caused by a low content of oxygen vacancies in the formation of Gd₂O₃–O₂. A different vacancy content may lead to different phase transition and formation processes of the Gd₂O₃.³⁸ The first stage of weak weight loss stops at 240 °C in the formation of Gd₂O₃–O₂. There are two strong exothermic peaks at 270 °C and 360 °C, which are attributed to the oxidation of organic matter and the conversion from Gd₂O₂S to crystalline Gd₂O₃.³⁹ In the presence of oxygen, the sulphur oxides are oxidized to oxy-sulphate compounds at the temperature range of 500 °C to 600 °C. The weak endothermic peak at 585 °C and the mass loss (2%) between 410 °C and 750 °C are probably due to simultaneous occurrence of exothermic and endothermic reactions with a lack of net heat to produce a thermal event.⁴⁰ The reaction could be shown using the following equation:⁴¹

2Gd₂O₂S + 4O_​2 → 2Gd₂O₂SO₄ → 2Gd₂O₃ + 2SO₂ + O₂ (3)

After 750 °C, there are no heat fluctuations or weight losses in either sample, which indicates the accomplishment of the decompositions and the formation of the Gd₂O₃ nanoparticles.

The Raman spectra of the two samples under 785 nm excitation are presented in Fig. 3. In the spectrum of the Gd₂O₃–O₂ sample, the bands located at 97, 118, 302 and 361 cm⁻¹ are attributed to the cubic structure with a B_g stretching vibration, the bands at 145, 239, and 447 cm⁻¹ are attributed to A_g stretching, and the bands at 402 and 561 cm⁻¹ are attributed to the A_g + B_g stretching.^42,43 In the spectrum of the Gd₂O₃–O₂-free sample, the peaks located at 70, 171, 250, 384, 483, 648 and 746 cm⁻¹ are attributed to the host monoclinic Gd₂O₃. The other peaks located at 80, 97, 110, 413, 443 cm⁻¹ arise from the cubic phase of Gd₂O₃.⁴⁴ The wavenumber values in Fig. 3 are a little different from the values reported in previous work. This may be caused by the phonon confinement effect in which nanoparticles can cause a frequency shift and the broadening of Raman peaks.

Fig. 3 Raman spectra of Gd₂O₃ nanoparticles which were calcined in different atmospheres.

Upconversion luminescence

The comparative analyses of the upconversion luminescence behaviours for the two samples are shown in Fig. 4, in which a 980 nm laser diode with a power of 100 mW was employed as the excitation source. The faint green emissions peaking at the ordinary centre of 523 nm and 538 nm come from the ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transitions of Er³⁺ ions, respectively. The dominating red emission originates from the Er³⁺: ⁴F_9/2 → ⁴I_15/2 transition.^6,45 We can clearly observe three sets of splitting peaks for all the transitions, and we have chosen to label them in order of H_1–4, S_1–3 and F_1–7, respectively. Apparently, the splitting transition probabilities of the two samples are totally different. This can be used as a fingerprint for identification of the lanthanides located in different crystal fields. The spectral profile of the Gd₂O₃–O₂ sample (blue lines in Fig. 4(a)) is confirmed in accordance with the Er³⁺ splitting peaks in the cubic lattice of Gd₂O₃.⁴⁶ The red lines of the Gd₂O₃–O₂-free sample indicate that most of the activated Er³⁺ ions have arisen from the monoclinic Gd₂O₃ lattice.⁴⁷ Importantly, the green emission is nearly eliminated while the red emission is more efficient from the monoclinic phase to cubic phase.

Fig. 4 (a) Room temperature upconversion luminescence spectra of Gd₂O₃:Yb³⁺,Er³⁺ under different calcination conditions. (b) Proposed energy transfer mechanisms showing the upconversion processes in Gd₂O₃:Yb³⁺,Er³⁺ nanoparticles under 980 nm laser excitation. Double-logarithmic plots of the excitation power dependent emission intensity of (c) the sample calcined in N₂, and (d) the sample calcined in ambient air. (e) The specific slope values of the different splitting transitions in the two samples.

To understand the upconversion mechanism, the excitation power (P) dependent upconversion (UC) emission intensity (I_up) of Gd₂O₃:Yb³⁺,Er³⁺ was measured. Fig. 4(c) and (d) demonstrate the dependence of I_up on P for Gd₂O₃:Yb³⁺,Er³⁺ under O₂-free and O₂ conditions, respectively. It is known that I_up is proportional to the power N of P, i.e.,⁴⁸

I_up ∝ P^N ⇒ log(I_up) = log(kP^N) = N log P + A, (4)

where N is the number of pump-photons required to populate the excited state and is determined by the slope of the fitted line. In Fig. 4(c)–(e), the slope values of 2.7, 2.7, 2.6 and 2.6 for the splitting states H_1–4 and 2.5, 2.5 and 2.4 for the splitting states S_1–3 are obtained, indicating that the population of the states ²H_11/2 and ⁴S_3/2 comes from the three-photon UC processes in the Gd₂O₃–O₂-free sample. The relatively smaller slopes that approach the value of 2 for H_1–4 and S_1–3 imply the corresponding two-photon UC processes in the Gd₂O₃–O₂ sample. The well-understood multiphoton population processes of Yb³⁺–Er³⁺ energy transfer are described in Fig. 4(b). This population difference is due to the distinct phonon energy for the two phases, e.g., the maximum phonon energy for the monoclinic phase (746 cm⁻¹) is much larger than the one for the cubic phase (561 cm⁻¹), as shown in Fig. 3. This induces a large probability of multiphonon relaxation for Er³⁺ in the monoclinic phase, via the following pathways: ⁴I_11/2⋯⁴I_13/2, ⁴F_9/2⋯⁴I_9/2⋯⁴I_11/2. For the red emission from the ⁴F_9/2 level, both of the samples have slopes corresponding to two-photon population upconversion. However, the cubic phase has a small slope for the ⁴F_9/2 → ⁴I_15/2 transition. This is due to the participation of cross relaxations including (⁴F_7/2 + ⁴I_11/2) → (⁴F_9/2 + ⁴F_9/2) and (²H_11/2 + ⁴I_15/2) → (⁴I_9/2 + ⁴I_13/2), which may have an excellent energy match in the cubic Gd₂O₃ sample.⁴⁹ The occurrence of the cross relaxations also helps us to understand why the cubic Gd₂O₃ phase has a more efficient population pathway (two-photon) for the ²H_11/2 and ⁴S_3/2 excited states of Er³⁺, but has limited green emission.

Thermal stability

The thermal sensitivities of the Gd₂O₃:Yb³⁺,Er³⁺ nanoparticles over a temperature range of 20–100 °C were studied using the fluorescence intensity. From Fig. 5(a)–(c), the red emission decreased when increasing the temperature from 20 to 100 °C, which was in accordance with the classical theory of thermal quenching.⁵⁰ Quantitative temperature dependent intensity analysis in Fig. 5(c) shows that the Gd₂O₃–O₂ sample had better thermal stability than the Gd₂O₃–O₂-free sample. The stability of the lanthanide-doped sesquioxides can be understood in terms of the phonon energy difference between the two phases. According to the multiphonon relaxation rate:⁵¹

W_mp(T) = W_mp(0)(1 − e^−hw_max/kT)^−p (5)

where p is the number of phonons and W_mp is the relaxation rate of 0 K, lower phonon energy leads to a lower multiphonon relaxation rate, and consequently to a higher thermal stability. It is suggested that the Gd₂O₃–O₂ sample is less sensitive to temperature, which is more beneficial for applications because it is little affected by the changing of external fields.

Fig. 5 Temperature-dependent luminescence of Gd₂O₃:Yb³⁺,Er³⁺ nanoparticles between 20 °C and 100 °C (excitation at 980 nm). Emission spectra for the sample (a) calcined in N₂, and (b) calcined in ambient air. (c) Relative intensity changing tendencies of the two samples with increasing temperature.

Conclusions

In conclusion, we successfully controlled the phase of Gd₂O₃ by calcining a Gd₂O₂S precursor in an O₂ limited atmosphere. The TG-DTA curves demonstrated the different formation of the Gd₂O₂S:Yb³⁺,Er³⁺ under different atmospheres, in which a high oxygen content can induce a pure cubic phase. Obvious splitting of the electronic transitions was observed in the Gd₂O₃ NPs, which emitted much stronger red light than green light. The cubic Gd₂O₃ phase is beneficial for the pure red upconversion, because of the lower maximum phonon energy, moderate multiphonon relaxations and cross relaxations. Also, the cubic Gd₂O₃ UC nanoparticles displayed a high thermal stability in terms of the luminescence intensity over a temperature range of 20–100 °C, which is more beneficial for application in multimodality biomedical imaging and NIR light-triggered photodynamic therapy.

Acknowledgements

The authors acknowledge financial support from Zhejiang Provincial Natural Science Foundation of China (No. LY14E020007, LQ14E020005, LY17E020007) and National Natural Science Foundation of China (No. 11404311, 11304202).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: X-ray pattern and transmission electron microscopy of Gd₂O₂S. See DOI: 10.1039/c6ra22754f

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