Optical thermometry of Er³⁺ in electrospun yttrium titanate nanobelts

Abstract

Er³⁺/Yb³⁺ co-doped Y₂Ti₂O₇ (YTOEY) nanobelts with a thickness of ∼100 nm are synthesized by electrospinning, which exhibit a high sensitivity and rapid responsiveness in temperature sensing. The fluorescence intensity ratio of two green intense signals that originated from the transitions of Er³⁺ ions can be developed for non-contact temperature measurements. Moreover, the nanoscale dimensions, porous structures and large surface area of the prepared YTOEY nanobelts allow the photothermal signals to pass through them quickly. The maximum absolute sensitivity at 513 K and the extreme relative sensitivity at 303 K are calculated to be 0.37% K⁻¹ and 1.17% K⁻¹, respectively, indicating that the nanobelts have attractive optical temperature sensing characteristics. These extraordinary results demonstrate that YTOEY nanobelts are a promising candidate for accurate temperature monitoring in thermo-sensitive equipment.

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1. Introduction

Non-contact optical thermometry has attracted widespread attention due to its wide operating temperature range, high spatial resolution and low dependence on the test environment, which can be applied to thermometry in electrical machinery, gas turbines, internal combustion engines and on-demand drug release.^1–6 Compared with the traditional contact method, non-contact optical temperature measurement based on the fluorescence intensity ratio (FIR) of two thermal coupling energy levels has the advantages of non-invasiveness, high precision and fast response.^7–11 Moreover, rare earth (RE) ion-doped materials have been widely applied in the field of non-contact optical thermometry because of their favorable temperature sensitivity, high upconversion efficiency, unique thermal stability, and excellent physicochemical stability.^12–20 Non-contact optical thermometry relies on appropriate energy separation in thermally coupled levels of RE, avoiding the overlap or the weakness of two emissions used for thermometry when the energy gap is unsuited. Among different RE ions, Er³⁺ ions used in non-contact optical thermometric sensing have received widespread attention, owing to their opportune energy separation of ∼700 cm⁻¹ between the thermally coupled levels ⁴S_3/2 and ²H_11/2.^21–23 For instance, L. Liu et al. reported the dependence of optical temperature sensing and photo-thermal conversion in β-NaYF₄:Yb³⁺,Er³⁺ nanoparticles, indicating that Er³⁺ ions have a promising future for application in temperature sensing.²⁴ Y. Yang et al. synthesized Yb³⁺/Er³⁺ codoped La₂O₂S phosphor for optical thermometry based on upconversion fluorescence, which demonstrates that the energy gap between the ²H_11/2 and ⁴S_3/2 levels of Er³⁺ ions is important for the sensitivity of the sensor.²⁵

Different kinds of upconversion materials have been prepared and widely used in the field of non-contact optical thermometry, such as ceramics, glass, phosphor and nanoparticles.^26–29 Compared with the abovementioned materials, one-dimensional (1D) nanobelt materials have attracted extensive attention due to their outstanding orientation, dimensional stability, large surface-to-volume ratio and high thermal sensitivity.^30–35 However, nanobelt materials with a high thermal sensitivity and rapid responsiveness for non-contact temperature sensing are rarely reported. Hence, an upconversion luminescent (UCL) material with a nanobelt structure is desired to be obtained and applied in the field of non-contact optical thermometric sensing. Among the UCL materials, Y₂Ti₂O₇ (YTO) with a pyrochlore structure has emerged as a bewitching matrix due to its relatively low phonon energy (∼712 cm⁻¹), elevated temperature heat-resistance, unique optical properties and potential application in non-contact optical temperature sensing.^36–39 Furthermore, electrospinning technology has been proved to be the most convenient and effective way to obtain blended materials.^40–42 Considering the latest development of thermal feedback materials, the direct synthesis of Er³⁺/Yb³⁺ co-doped Y₂Ti₂O₇ (YTOEY) nanobelts with excellent UCL and optical thermal sensitivity using the electrospinning method is very attractive.⁴³

In this paper, a series of Er³⁺/Yb³⁺ co-doped YTOEY nanobelts as promising optical temperature sensing materials are prepared via the electrospinning method, and the thickness of nanobelts is determined to be ∼100 nm. Besides, the upconversion emission spectra are analyzed and the possible mechanism of UCL is elucidated through power dependence. For the non-contact temperature sensing application, as the temperature can be determined according to the FIR of the two thermally coupled energy levels ²H_11/2 and ⁴S_3/2 emission originated from Er³⁺ ions, the thermal sensing characteristics of YTOEY nanobelts were analyzed in the temperature range of 303 K to 693 K. Fig. 1 indicates the potential application of YTOEY nanobelts as heat source monitors which can be easily integrated into multifunctional nanochips for high-sensitivity and high-speed non-contact temperature sensing.

Fig. 1 Potential application of YTOEY nanobelts in non-contact temperature sensing.

2. Experimental

2.1 Fabrication of (Y_1−x−yEr_xYb_y)₂Ti₂O₇ nanobelts

Er³⁺/Yb³⁺ co-doped Y₂Ti₂O₇ nanobelts were synthesized using an electrospinning technique.^44,45 High-purity powders of Y₂O₃ (4N), Er₂O₃ (4N), and Yb₂O₃ (4N) were used as the raw materials. First of all, Y₂O₃, Yb₂O₃ and Er₂O₃ powders were weighed, dissolved in concentrated HNO₃ (AR), and transformed into nitrate Ln(NO₃)₃·6H₂O (Ln³⁺ = Y³⁺, Yb³⁺, and Er³⁺) by evaporation crystallization. Different from [Y_0.918(Ho, Yb)_0.082]₂Ti₂O₇ adopted in ref. 46, a lower concentration of rare earth was used to reduce the formation of the miscellaneous phase considering the effectiveness of Er³⁺ luminescence. The chemical formula of nanobelts is (Y_1−x−yEr_xYb_y)₂Ti₂O₇, where x = 0.0182m, y = 0.00182n, and (m, n) = (0, 0), (1, 0),(2, 0), (4, 0), and (10, 0) for Er³⁺ single-doping and (m, n) = (2, 2) and (2, 6) for Er³⁺ and Yb³⁺ co-doping cases labelled as YTOE_mY_n. For a better representation, the corresponding micron-belt samples are abbreviated as YTOE₀Y₀, YTOE₁Y₀, YTOE₂Y₀, YTOE₄Y₀, YTOE₁₀Y₀, YTOE₂Y₂, and YTOE₂Y₆. In specific experiments, Ln(NO₃)₃·6H₂O (Ln³⁺ = Y³⁺, Yb³⁺, and Er³⁺) with a total amount of 2.74 mmol was slowly dissolved in a completely mixed solution composed of 0.85 mL Ti(C₄H₉O)₄ (TBOT) (CP) and 2 mL acetic acid (HAC) (AR), which was marked as 1#. At the same time, 0.48 g of polyvinylpyrrolidone (PVP, M_w = 1 300 000) was fully mixed with 3.6 mL of N,N-dimethylformamide (DMF) (AR) and thoroughly stirred for 3 h, which was labeled as 2#. Shortly, the abovementioned 1# and 2# solutions were mixed and the spinning solution was obtained under strong magnetic stirring for 12 hours. In the whole electrospinning process, the working voltage was 9 kV DC, the pushing speed was always kept at 0.3 mL h⁻¹ and the distance between the receiving plate and the spinneret was 15 cm. In the end, the as-spun belts were dried in a desiccator at room temperature for 12 h, then heated from room temperature to 800 °C in a muffle furnace at a rate of 1 °C min⁻¹, kept at 800 °C for 8 h and finally cooled down to room temperature at the same rate for other characterization studies. The preparation process of the YTOEY nanobelts is illustrated in Fig. 2.

Fig. 2 Fabrication process of YTOEY nanobelts by electrospinning.

2.2 Measurements and characterization

The X-ray diffraction pattern of the nanobelts was examined using a Shimadzu XRD-7000 X-ray diffractometer using Cu Kα radiation (λ = 1.5406 Å) with a step-by-step scanning method over the 2θ range from 10° to 80°. The elemental and morphological analyses of the as-synthesized nanobelts were performed using an energy dispersive spectrometer (EDS) and a scanning electron microscope (JEOL JSM-7800F SEM), respectively. The upconversion spectra at room temperature were recorded using a Hitachi F-7000 spectrophotometer equipped with a 977 nm fiber-pigtailed multimode diode laser. The upconversion luminescence spectra at different temperatures were obtained by using a Hitachi F-4600 spectrophotometer excited by a 980 nm fiber-pigtailed diode laser (MW-GX-980/5000 mW). The temperature control system was used to control (DMU-TC450) the test temperature of the sample in the range of 303 to 693 K.

3. Results and discussion

3.1 Structure and morphology

Powder XRD was adopted to identify the crystal structures of the prepared samples. As illustrated in Fig. 3(a), a series of characteristic crystal surface indices (hkl) were recorded for different samples, which are consistent with the reference YTO with a face-centered cubic pyrochlore structure (JCPDS file number: 42-0413), indicating that the samples contain a pure crystalline phase.^47–50Fig. 3(b) shows a partially enlarged image of the diffraction position of the 2θ shifting in the (222) crystal plane corresponding to Fig. 3(a). It can be seen that the position of the main peak slightly shifts to the right when introducing the doping of Er³⁺ and Yb³⁺ ions. The reason for this phenomenon is that part of the Y³⁺ ions (r = 1.155 Å) with a larger crystal radius are replaced by Er³⁺ (r = 1.14 Å) and Yb³⁺ (r = 1.12 Å) with smaller crystal radii, which leads to lattice shrinkage.^51,52 Meanwhile, the slight shift proves that two RE ions partially replace Y³⁺ ions in the matrix lattice successfully.

Fig. 3 (a) XRD patterns of YTOE₂Y₆ and YTOE₀Y₀ nanobelts after heat treatment. (b) Local amplification of XRD patterns on the (222) plane.

The schematic diagram of the crystal structure of a single Y₂Ti₂O₇ cell is illustrated in Fig. 4. In the cubic pyrochlore structure of Y₂Ti₂O₇ (A₂B₂O₇), the larger cation is 8-coordinated at the A (16d) position and the smaller cation is 6-coordinated at the B (16c) position. Besides, the oxygen atoms occupy different crystal axes: 48f(O₁), 8b(O₂) and 8a(O₃). The 48f(O₁) site is coordinated with two Y and two Ti ions, the 8b(O₂) site is coordinated with four Y ions, and the third anionic site in the united cell, 8a(O₃), is coordinated with four Ti cations, which is usually found to be an empty and incomplete ordered pyrochlore lattice. Furthermore, owing to the similar crystal radii of Yb³⁺, Er³⁺ and Y³⁺, some Y³⁺ ions in the lattice are gradually replaced by Er³⁺ or Yb³⁺ ions. Rietveld refinement of the YTOE₂Y₆ sample was performed, and the subtle crystal microstructure details are depicted in Fig. 5. In addition, accurate lattice parameters and the results of Rietveld refinement are listed in Table 1 for the YTOE₂Y₆ sample. Accordingly, it can be further testified that the synthesized samples have phase purity using Rietveld refinement.

Fig. 4 Schematic diagram of a unit cell of the YTO crystal structure.

Fig. 5 XRD profiles for the Rietveld refinement results of YTOE₂Y₆ crystal particles.

Table 1 Rietveld refinement and lattice parameters of YTOE₂Y₆

Formula	YTOE2Y6
Space group	Fd3̄m
a = b = c (Å)	10.1244
α = β = γ (°)	90°
Z	8
V (Å^3)	1029.87
R p (%)	8.69
R wp (%)	11.99
χ ^2	3.762

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The surface morphologies of YTOE₂Y₆ nanobelts and dimension information were observed using FE-SEM, as shown in Fig. 6. After heat treatment at 800 °C for 8 h, the length, width and thickness of the sample were found to be ∼40 μm, ∼4 μm and ∼100 nm, respectively, as presented in Fig. 6. Therefore, the photothermal signal can completely pass through the belts due to the nanometer-sized thickness, and the larger superficial area using in temperature detection can synchronously respond to the thermal field in the region because nanobelts are composed of only a few tiers of precipitated YTOEY crystal particles. At the same time, it can be observed that the surface of the YTOE₂Y₆ nanobelts has a porous structure, which is caused by the decomposition of PVP during high-temperature heat treatment. As illustrated in Fig. 7, considering YTOE₂Y₆ nanobelts as an example, the elemental composition was investigated using EDS. Herein, Er, Yb, Y, Ti and O elements were observed and were distributed uniformly in the scanning region. The results of the above discussion are also consistent with the results of the XRD analysis, indicating that YTOEY nanobelts can be successfully synthesized using the electrospinning method.

Fig. 6 (a) SEM micrographs of YTOE₂Y₆ nanobelts after heat treatment. (b) Frontal morphology of the nanobelts. (c and d) Cross-sectional morphology of the nanobelts.

Fig. 7 (a) Electron image of YTOE₂Y₆ nanobelts. (b–f) EDS spectra of elements. (g) EDS spectrum of YTOE₂Y₆ nanobelts.

3.2 Upconversion luminescence

To further comprehend the mechanism of UCL, the upconversion emission spectra of YTO nanobelts doped with different concentrations of RE ions were measured using a 977 nm near-infrared pump laser. As illustrated in Fig. 8, UCL was recorded by using a 977 nm laser with a 630 mW pump power to excite the belts with different doping concentrations after heat treatment, confirming that YTOEY nanobelts are highly efficient upconversion materials. The spectra of UCL exhibit two green (∼524 nm and ∼546 nm) emission bands and one red (∼656 nm) emission band when excited using a 977 nm laser. The two green emission bands originated from the ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transitions and the red band is attributed to the combination of ⁴F_9/2 → ⁴I_15/2 transitions.^53–55 The bimodal phenomena of green and red emission bands were caused by the Stark splitting of erbium degenerated energy levels.⁵⁶ The relationship between the UCL intensities and the different concentrations of Er³⁺ ions is illustrated in the inset of Fig. 8. Meanwhile, the upconversion emission intensity shows an increasing trend first and then a decreasing trend with the gradual increase of the Er³⁺ ion doping ratio and the optimal doping ratio is YTOE₂Y₀. Furthermore, the co-doping ratio of Yb³⁺ and Er³⁺ ions has a strong effect on the UCL intensity of the nanobelts. In the absence of Yb³⁺ ions, the photon energy of the 977 nm laser cannot effectively excite the energy level of Er³⁺ ions, so the intensity of UCL is relatively weak. However, bright green UCL emissions located at ∼524 and ∼546 nm can be clearly observed when Yb³⁺ ions are doped into the YTO nanobelts as shown in Fig. 8, which indicates that there is a more effective energy transfer between Er³⁺ and Yb³⁺ under 977 nm laser excitation.^57–60

Fig. 8 UCL spectra of YTOEY nanobelts with different doping ratios. Inset: Partially enlarged upconversion spectra of YTO nanobelts doped with different Er³⁺ concentrations.

As shown in Fig. 9, to further elucidate the mechanism of UCL in YTOEY nanobelts, the relationship between pump power and the upconversion emission intensity of the belts was investigated. In the experimental range of 160 to 630 mW, the intensity of the main emission peak (∼546 nm) increases rapidly with a slow increase in pump power. The pump power dependence of the two green (∼524 and ∼546 nm) luminescence bands and one red (∼656 nm) luminescence band is shown in the inset of Fig. 9. Furthermore, for the UCL process, the intensity of emitted radiation (I) is proportional to the exponent (n) of pump power (P), and their relationship can be described using the well-known formula I ∝ Pⁿ, where n is the number of near-infrared photons absorbed by the sensitizer-activator coupling during the excitation.⁶¹ The inset of Fig. 9 demonstrates the fitting line and experimental data of YTOE₂Y₆ nanobelts and indicates that the fitted n values are 1.89, 1.91 and 1.78 for the two green emission band and one red emission band, respectively. These fitting values are close to 2, revealing that all three UCL emission bands mentioned above are two-photon processes. Meanwhile, the fitting values are slightly less than 2, indicating that somehow a quenching process originated from the thermal effect in the process of UCL emission.⁶²

Fig. 9 UCL spectra of YTOE₂Y₆ nanobelts under excitation using a 977 nm laser. Inset: Dependence of UCL emission intensities on pump powers in YTOE₂Y₆ nanobelts.

The schematic energy level diagram of UCL emission in YTOEY nanobelts under 977 nm laser excitation is illustrated in Fig. 10 and energy transfer (ET), excited-state absorption (ESA) and ground-state absorption (GSA) can be observed intuitively. As mentioned above, Yb³⁺ ions have a broader and larger absorption cross-section than Er³⁺ ions, and there is a perfect resonance between the ⁴I_11/2 level of Er³⁺ ions and the ²F_5/2 level of Yb³⁺ ions. Consequently, there will be an effective resonance energy transfer from Yb³⁺ to Er³⁺ ions.⁶³ First of all, the electrons of the ⁴I_15/2 level of Er³⁺ ions are excited to the ⁴I_11/2 level via ET or GSA of the adjacent Yb³⁺ ions. Part of the excited electrons in the ⁴I_11/2 level relax to the ⁴I_13/2 level. The rest of the excited electrons in the ⁴I_11/2 level are further excited to the ⁴F_7/2 level via ESA absorption of the exciting photon energy. Subsequently, the excited electrons at the ⁴F_7/2 level relax to the ²H_11/2 and ⁴S_3/2 levels through non-radiation, and then electrons at the ²H_11/2 and ⁴S_3/2 levels transfer to the ground state level ⁴I_15/2 through radiation and emit green UCL at ∼524 nm and ∼546 nm, respectively. Meanwhile, the ions excited to the ⁴I_13/2 level also absorb the photon energy from outside and further excite to the ⁴F_9/2 level. Then, the radiation transition from ⁴F_9/2 to ⁴I_15/2 produces a weak red UCL at ∼656 nm. Therefore, according to the above description, it can be concluded that each emission process at ∼524, ∼546 and ∼656 nm is a two-photon process in this work.

Fig. 10 Energy level diagram of YTOEY nanobelts excited using a 977 nm laser.

3.3 Temperature sensing behavior of YTOEY nanobelts

The application of Er³⁺-doped materials for non-contact temperature measurement is one of their most representative applications. Hence, the non-contact optical temperature-sensitive behavior of YTOE₂Y₆ nanobelts was investigated using a mature FIR system under 980 nm laser excitation at a 650 mW power. For this purpose, the up-conversion emission spectra of the YTOE₂Y₆ nanobelts with temperature ranging from 303 to 693 K are also examined as exhibited in Fig. 11, and a schematic illustration of the optical temperature sensing test is shown in Fig. 11(a). In Fig. 11 (b–d), as the test temperature increases slowly, the upconversion emission intensity at ∼546 nm (⁴S_3/2 → ⁴I_15/2) shows an overall downward trend with a slight upward trend at 450 K; nevertheless, the emission intensity located at ∼524 nm (²H_11/2 → ⁴I_15/2) presents trifling continuous fluctuations. Changes of the emission intensity from ²H_11/2 and ⁴S_3/2 levels show a nearly opposite trend with temperature increase, which originated from the evolution of the population redistribution.

Fig. 11 (a) Schematic illustration of temperature sensing measurement. (b) Temperature-dependent UCL spectra. (c) Histogram displaying the integrated intensities. (d) Emission contour mapping of upconversion at different temperatures.

The thermal sensitivity of YTOE₂Y₆ nanobelts was examined by using the FIR technique with two thermally coupled levels of Er³⁺ ions. Typically, the small energy gap between ²H_11/2 and ⁴S_3/2 levels allows the thermalized population of the upper ²H_11/2 level at the expense of the lower ⁴S_3/2 level. Thus, the populations of the two close levels, ²H_11/2 and ⁴S_3/2, follow Boltzmann distribution, which can be determined by using the following equation:⁶⁴

where I is the fluorescence intensity, σ is the stimulated emission cross-section, g is the degeneracy and ω is the angular frequency of the ²H_11/2 and ⁴S_3/2 levels, respectively. Meanwhile, k is the Boltzmann constant, ΔE is the energy gap between ²H_11/2 and ⁴S_3/2 levels, and T is the absolute temperature. Based on eqn (1), Fig. 12(a) shows the dependence of FIR technology on the YTOE₂Y₆ nanobelts at different test temperatures and the fitted values of C and ΔE/k are 7.41 and 1074.33 K, respectively. At the same time, a high correlation coefficient of 0.9995 was obtained, which shows that the fitting effect is in line with the expectation. Consequently, the temperature can be accurately determined based on the ratio of UCL intensity between ²H_11/2 and ⁴S_3/2 energy levels.

Fig. 12 FIR of (I_524 nm/I_546 nm) (a) and absolute sensitivity S_a and relative sensitivity S_r (b) of YTOE₂Y₆ nanobelts in the measurement temperature range of 303–693 K.

In a practical application of temperature measurement, absolute sensitivity (S_a) and relative sensitivity (S_r) are two important indexes to evaluate the performance of the optical thermometric sensor. S_a can be defined as follows:⁶⁵

Meanwhile, S_r assesses the non-contact optical thermometric sensing performance, which can be expressed using the following equation:

Fig. 12(b) shows the calculated plots of S_a and S_rversus temperature in the range from 303 to 693 K. The maximum S_a value is about 0.37% K⁻¹ at 513 K and the maximum S_r value is about 1.17% K⁻¹ at 303 K. Furthermore, the maximum S_a and S_r values of several reported RE-doped materials are listed in Table 2. Compared with other Er³⁺/Yb³⁺ ion doped materials, YTOE₂Y₆ nanobelts have both a wider response range and a higher sensitivity. Moreover, the prominent S_a and S_r values of YTOEY nanobelts indicate that they have great potential to be used as non-contact optical thermometric sensors.

Table 2 Maximum sensitivity values (S_a and S_r) of different Er³⁺/Yb³⁺ co-doped materials in different temperature ranges

Host lattice: Er^3+/Yb^3+ ions	Temperature range (K)			S r (K^−1)	S a (%K^−1)		Ref.
Al(PO3)3-BaF2-SrF2: Er^3+/Yb^3+		100–280	542/T^2			0.15	66
Ba5Gd8Zn4O21: Er^3+/Yb^3+		298–573	629.69/T^2			0.24	67
TeO2–WO3 glass: Er^3+/Yb^3+		300–690	976.89/T^2			0.28	29
Silicate glass: Er^3+/Yb^3+		296–723	592.6/T^2			0.33	68
Na0.5Bi0.5TiO3: Er^3+/Yb^3+		163–613	827.26/T^2			0.31	69
Y2Ti2O7: Er^3+/Yb^3+		303–693	1074.33/T^2			0.37	This work
β-NaYF4: Er^3+/Yb^3+		293–753	1113/T^2			0.39	70
Y2O3: Er^3+/Yb^3+		93–613	868.08/T^2			0.44	71
BCZT ceramics: Er^3+/Yb^3+		173–573	1234.2/T^2			0.68	26
Ba0.5Sr0.5TiO3: Er^3+/Yb^3+		288–538	1240.3/T^2			0.76	72
Bi4Ti3O12: Er^3+/Yb^3+		293–523	959.33/T^2			1.02	73

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4. Conclusions

YTOEY nanobelts with a thickness of ∼100 nm were fabricated by electrospinning, which were investigated for their spectral characteristics and temperature sensing behavior. Two intense green emission bands located at ∼524 and ∼546 nm were attributed to the ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transitions of Er³⁺ ions under excitation using a 977 nm laser, which is conducive to the non-contact temperature sensing application. Furthermore, the maximum of absolute sensitivity S_a was calculated to be 0.37% K⁻¹ at 513 K and a relative sensitivity S_r value of 1.17% K⁻¹ is obtained at 303 K, indicating the promising application of YTOEY nanobelts in non-contact optical thermometric sensing. Therefore, the excellent sensitivity and rapid responsiveness of optical temperature sensing demonstrate the potential of YTOEY nanobelts to be used as optical thermosensitive materials in non-contact temperature monitoring.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

The research work was supported by the Scientific Research Funding Project from the Educational Department of Liaoning Province, China (Grant No. J2020047) and the Research Grants Council of the Hong Kong Special Administrative Region, China (Grant No. CityU 11219819).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0nj05478j

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